Sine-cosine optical frequency detection devices for photonics integrated circuits and applications in lidar and other distributed optical sensing

ABSTRACT

The disclosed technology can be implemented in photonics integrated circuit (PIC) to provide an optical frequency detection device for measuring an optical frequency of light using two Mach-Zehnder interferometer where the delay imbalance in the first interferometer is configured to be one quarter wavelength longer than that of the second interferometer to produce an additional phase difference between the two arms. The two outputs of each interferometer are then detected by two photodetectors to produce two complementary interference signals. The difference between the two complementary interference signals of the first interferometer is a sine function of the optical frequency while the difference between the two complementary interference signals of the second interferometer is proportional to a cosine function of the optical frequency. Using the sine/cosine interpretation algorithm commonly used for the rotation encoders/decoders, any increments in optical frequency can be readily obtained.

PRIORITY CLAIMS AND RELATED PATENT APPLICATIONS

This patent document is a continuation-in-part application of and claimsthe benefit of priority to U.S. patent application Ser. No. 17/515,050,entitled “SINE-COSINE OPTICAL FREQUENCY DETECTION DEVICES FOR PHOTONICSINTEGRATED CIRCUITS AND APPLICATIONS IN LIDAR AND OTHER DISTRIBUTEDOPTICAL SENSING” and filed on Oct. 29, 2021, and published as U.S.Patent Application Publication No. US 2022/0137298 A1 on May 5, 2022.U.S. patent application Ser. No. 17/515,050 further claims priorities toand benefits of (1) U.S. Provisional Application No. 63/163,500, filedon Mar. 19, 2021, and (2) U.S. Provisional Application No. 63/108,175,filed on Oct. 30, 2020. The entire contents of the before-mentionedpatent applications are incorporated by reference as part of thedisclosure of this application.

TECHNICAL FIELD

This patent document relates to photonics integrated circuit devicescapable of measuring optical frequency information of light and the usesof such devices in various applications including light detection andranging (LiDAR) and others.

BACKGROUND

Many applications involve determining the optical frequency informationof light. In some of those applications, it can be important toprecisely measure or obtain the optical frequency information with ahigh resolution and at a high speed. Examples of such applicationsinclude laser frequency control, laser frequency analysis, fiber Bragggrating interrogation, frequency trigger signals for optical coherencetomography, optical frequency domain reflectometry, and chirped LiDARsystems for collision prevention for autonomous driving vehicles.

SUMMARY

In one aspect, the disclosed technology can be implemented in photonicsintegrated circuit (PIC), specifically with silicon photonics, toprovide an optical frequency detection device for measuring an opticalfrequency of light to include an input port that receives light at anoptical frequency to be measured; an optical coupler to split the lightfrom the input port into two branches. Light in each branch then entersa Mach-Zehnder interferometer with a certain delay imbalance between thetwo arms. By design, the delay imbalance in the first interferometer isconfigured to be one quarter wavelength longer than that of the secondinterferometer to produce an additional phase difference between the twoarms. The two outputs of each interferometer are then detected by twophotodetectors to produce two complementary interference signals. Thedifference between the two complementary interference signals of thefirst interferometer is a sine function of the optical frequency whilethe difference between the two complementary interference signals of thesecond interferometer is proportional to a cosine function of theoptical frequency. Using the sine/cosine interpretation algorithmcommonly used for the rotation encoders/decoders, any increments inoptical frequency can be readily obtained.

In another aspect, an optical frequency detection device can beconstructed by adding a third Mach-Zehnder interferometer with a muchsmaller delay imbalance to the above two Mach-Zehnder interferometer onthe same chip to produce a much slow output variation when the opticalfrequency is changed. Collectively with all three interferometers, theabsolute optical frequency can also be determined.

In another aspect, the disclosed technology can be implemented toprovide a device for measuring an optical frequency of light to includea substrate and first optical waveguides integrated to and supported bythe substrate and coupled to form a first Mach-Zehnder interferometerhaving two interfering optical arms and an input optical port to receivea first portion of input light at an input optical wavelength that issplit into the two interfering optical arms and an output optical portto receive and combine light from the two interfering optical arms toproduce two first optical output interferometer signals; two firstphotodetectors supported by the substrate and located to receive the twofirst optical output interferometer signals, respectively. The two firstphotodetectors produce first and second detector signals, respectively,and each of the first and second detector signals varies as a sinefunction of an optical frequency corresponding to the input opticalwavelength. This device also includes second optical waveguidesintegrated to and supported by the substrate and coupled to form asecond Mach-Zehnder interferometer having two interfering optical armsand an input optical port to receive a second portion of the input lightwhich is split into the two interfering optical arms, and an outputoptical port to receive and combine light from the two interferingoptical arms to produce two second output interferometer signals and thesecond Mach-Zehnder interferometer is structured to have a phasedifference between the two interfering arms different by one quarter ofthe input optical wavelength from a phase difference between the twointerfering arms of the first Mach-Zehnder interferometer. This devicefurther includes two second photodetectors supported by the substrateand located to receive the two second optical output interferometersignals, respectively, wherein the two second photodetectors producethird and fourth detector signals, respectively, and wherein each of thethird and fourth detector signals varies as a cosine function of theoptical frequency corresponding to the input optical wavelength; and aprocessing module coupled to receive the first, second, third and fourthdetector signals and operable to process the first, second, third andfourth detector signals to determine a change in the optical frequencyof the input light.

In yet another aspect, the disclosed technology can be implemented toprovide a device for measuring an optical frequency of light to includea substrate; a beam splitter supported by the substrate and located inan optical path of the input light to split the input light into a firstinput beam of the input light and a second input beam of the inputlight; and a first optical coupler supported by the substrate andlocated to receive the first input beam and to split the first inputbeam into the first and second portions of the input light. This deviceincludes first optical waveguides integrated to and supported by thesubstrate and coupled to form a first Mach-Zehnder interferometer havingtwo interfering optical arms and an input optical port to receive thefirst portion of input light and split the received first portion intodifferent beams in the two interfering optical arms and an outputoptical port to receive and combine light from the two interferingoptical arms to produce three or more first optical outputinterferometer signals in different phases relative to one another;|three or more first photodetectors supported by the substrate andlocated to receive the three or more first optical output interferometersignals, respectively. The first photodetectors produce three or morefirst detector signals, respectively, and each of the first detectorsignals varies as a sine or cosine function of an optical frequencycorresponding to the input optical wavelength. This device also includessecond optical waveguides integrated to and supported by the substrateand coupled to form a waveguide device to receive the second input beamfrom the beam splitter and to produce two output signals ofcomplementary wavelength responses; and two second photodetectorssupported by the substrate and located to receive the two output signalsof the waveguide device, respectively. The two second photodetectorsproduce two second detector signals, respectively, and wherein each ofthe second detector signals varies as a cosine function of the opticalfrequency corresponding to the input optical wavelength. A processingmodule is further included in this device and is coupled to receive thefirst and second detector signals and operable to process the first andsecond detector signals to determine an absolute value of, and a changein, the optical frequency of the input light.

The above devices and PIC optical frequency detectors based on thedisclosed technology can be implemented to enable on-chip integration ofcoherence Lidar, optical coherence tomography (OCT), optical frequencydomain reflectometer (OFDR), and fiber Bragg grating (FBG) interrogatorrequiring fast and precise optical frequency detection. In addition,precise laser frequency control with arbitrary waveforms can also beimplemented with the optical frequency detector disclosed in thisapplication.

The above and other aspects and their implementations of the disclosedtechnology are described in greater detail in the drawings, thedescription and the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 includes FIGS. 1 a and 1 b illustrating an example of a prior artk-clock (f-clock) generator based on a Mach-Zehnder interferometer forgenerating equally-spaced frequency markers. FIG. 1 a shows an exampleof a prior art incremental frequency detector using a Mach-Zehnderinterferometer of a large delay imbalance between the two arms. Such aninterferometer may also suffer polarization fading caused by thepolarization variations in the long fiber delay. FIG. 1 b shows therelationship between measurement range and frequency measurementresolution. In principle, for the unambiguous detection of frequencyincrement, the detection range is limited to half of the free spectralrange (FSR). However, a larger FSR results in a lower frequency slopeand therefore worse measurement resolution. Conversely, a betterfrequency measurement resolution requires small FSR or a large delayimbalance between the two arms. For detecting a unidirectional opticalfrequency sweep, one may simply get zero-crossing frequencies which haveequal frequency spacing for certain applications.

FIG. 2 is an example of a sine/cosine optical frequency detector (OFD).Light inputting to the frequency detector is split into two branches,with each of them entering a Mach-Zehnder interferometer. The topinterferometer has a delay imbalance of ΔL between two optical brancheswithin the interferometer, while the bottom interferometer has a delayimbalance of ΔL+/−λ/4 between two optical branches within theinterferomter. Considering the fabrication tolerances, in someimplementations, the nominal delay imbalances of the two interferometerscan be made the same and the λ/4 difference between imbalances of thetwo interferometers can be induced by a π/2 phase shifter. An optionalphase turning device such as an optical heater may be implemented in atleast one of the two optical arms to fine tune the interferometer. Suchan OFD may be integrated onto a photonics integrated circuit (PIC) overa substrate where optical waveguides are used to guide light fromcomponent to component.

FIG. 3 is an example of a polarization insensitive sine/cosine opticalfrequency detector. Input light is separated by a polarization beamsplitter (PBS) into two separate beams in TE and TM modes that aredirected into TE and TM branches of waveguides, respectively. The TEbranch is coupled to a frequency decoder made with two Mach-Zenhderinterferometers described in FIG. 2 where an optical beam splitter,e.g., a multi-mode interferometer (MIMI) device, is used to split the TEmode light in the TE branch into two separate beams in the frequencydecoder. In the TM branch, the TM mode is converted to TE mode with amode converter into a TE waveguide before entering a second frequencydetector made of two Mach-Zenhder interferometer described in FIG. 2 .Under this design, no matter what the input polarization is, sufficientoptical power can be present in either the upper or lower branch of thedevice for obtaining the incremental optical frequency information. Inother implementations, the PBS and the TM to TE converter may bereplaced with a polarization splitter and rotator (PBSR).

FIG. 4 is an example of an analog circuit for calculating the instantphase or frequency of the light signal.

FIG. 5 includes FIGS. 5 a and 5 b and shows an example of a digitalcircuit for obtaining the instant phase or frequency of the lightsignal. The comparators in FIG. 5 a are to get the zero crossingpositions for period counting, as shown in the top and middle graphs inFIG. 5 b . The ADC in FIG. 5 a is to digitize the data and then the DSPis used to calculate the instant phase using θ=tan⁻¹(sinY2/cosY1), asshown in the bottom graph in FIG. 5 b.

FIG. 6 is an example of an embodiment of the sine/cosine opticalfrequency detector (OFD) based on the disclosed technology in thispatent document, which combines a Mach-Zehnder interferometer (MZI) of alarge free spectral range (FSR) (FSR₂=c/δL) with a pair of MZIs of asmall FSR (FSR₁=c/ΔL) to enable absolute optical frequency detection.Input light is split by an input MMI into two beams guided by twowaveguides. The upper waveguide branch goes to the OFD described in FIG.2 and the lower waveguide branch goes to another Mach-Zehnderinterferometer with a much smaller delay imbalance of δL of the twooptical arms that provides a much larger free spectral range (FSR₂).

FIG. 7 is an example for using the combination of an optical frequencydetector (OFD) of a large FSR an OFD of a small FSR described in FIG. 6for making an absolute frequency detector. The value of the large FSR ischosen such that in the frequency (or wavelength) range of interest, thedetected optical power change is limited in the first quarter of thecosine function for the coarse absolute optical frequency measurement,as shown in the figure. Within this quarter period, there is a largeamount of periods produced by the OFD of the small FSR. So long as thefrequency resolution of the OFD with the large FSR is sufficiently fineto resolve a period of the OFD of the small FSR, the absolute frequencyof the light source can be obtained unambiguously.

FIG. 8 is an example of an embodiment of the sine/cosine opticalfrequency detector capable of performing absolute optical frequencydetection based on combining a pair of Mach-Zehnder interferometers of asmall free spectral range (FSR₁=c/δL) with a directional coupler basedwavelength division multiplexer (WDM) to enable absolute opticalfrequency detection. Input light is polarized (e.g., in the TEpolarization mode) and is split by an input MMI into two TE polarizedbeams that are directed into two waveguides. The upper waveguide branchis coupled the OFD described in FIG. 2 while the lower waveguide branchis coupled to a coarse WDM made with a directional coupler with a largefree-spectral range (FSR₂).

FIG. 9 includes FIGS. 9A, 9B and 9C. FIG. 9A is an example of anembodiment of the sine/cosine optical frequency detector which combinesan interferometer of a small FSR (FSR₁=c/ΔL, e.g., on the order of GHz)made and a MZI with a much larger FSR (FSR₂=c/δL, e.g., on the order ofthousands GHz) for absolute optical frequency detection. FIG. 9B showsexamples of outputs from the three photodetectors of the 2×3 MMI couplerin FIG. 9A. FIG. 9C illustrates an example of another implementation ofthe sine/cosine optical frequency detector in FIG. 9A where adirectional coupler based WDM is used to replace the MZI device with thelarger FSR.

FIG. 10 is an example of an embodiment of a sine/cosine opticalfrequency detector using a 2×4 MIMI coupler to replace the 2×3 MIMIcoupler in the sine/cosine optical frequency detector in FIG. 9A. Thealternative implementation in FIG. 9C can also be used to implement thedevice in FIG. 10 .

FIG. 11 shows an example of a configuration for minimizing polarizationsensitivity of a sine/cosine optical frequency detector disclosed inthis patent document. In general SiO₂ based photonics integratedcircuits (PIC) are not polarization sensitive, while Si and SiN basedPICs may be polarization sensitive. The incoming lightwave with anarbitrary polarization is split by a polarization splitter and rotator(PB SR) with the TM polarization rotated into the TE polarization. Twoidentical OFD's described FIG. 8-10 are included to simultaneouslydetect the optical frequency in the two arms to eliminate thepolarization sensitivity, similar to the design in FIG. 3 .

FIG. 12 is an example of a tunable laser integrated with any of thesine/cosine optical frequency detectors disclosed above on a PIC chip.The detected frequency can be fed back to control the laser frequency asan optical frequency synthesizer. This device can be used to 1)stabilize the laser output to a fixed frequency in the range; 2)generate a frequency ramp with a high linearity; or 3) generate anarbitrary frequency variation waveform.

FIG. 13 is an example of an optical coherence domain reflectometer(OFDR) integrated with a tunable laser, a pair of balancedphotodetectors (PD5 and PD6 with an optional variable optical attenuator(VOA) in front of at least one photodetector), and a sine/cosine opticalfrequency detector integrated on a PIC chip based on the disclosure ofthis patent document. In an OFDR system, the precisely known frequencyincrement is needed for obtaining the accurate distance information ofthe object. An optical coherence domain reflectometer (OFDR) may beimplemented by using an optical interferometer (e.g., a Mach-Zenhderinterferometer as shown in FIG. 1 or Michelson interferometer) with alarge delay imbalance (e.g., on the order of 100 meters in some cases)to generate f-clock representing the frequency increment for triggeringthe data acquisition. Such a large interferometer is difficult to beintegrated on a PIC chip. The OFD in FIG. 13 can use the laser frequencycontrol based on the detection by the OFD to obtain the same frequencyresolution with only few mm delay imbalance and therefore enable thedirect integration on a PIC chip. Alternatively, the obtained frequencyinformation may also be used to control the tunable laser for generatingultra-linear frequency modulation.

FIG. 14 is an example of a chirped LiDAR integrated with a tunablelaser, a pair of balanced photodetectors (PD5 and PD6 with an optionalvariable optical attenuator (VOA) in front of at least onephotodetector), and a sine/cosine optical frequency detector integratedon a PIC chip based on the disclosure of this patent document. In such aLiDAR system, the precisely known frequency increment is needed forobtaining the accurate distance information of the object. A LiDAR maybe implemented by using an optical interferometer (e.g., a Mach-Zehnderinterferometer as shown in FIG. 1 or Michelson interferometer) with alarge delay imbalance (e.g., on the order of 100 meters in some cases)to generate f-clock representing the frequency increment for triggeringthe data acquisition. Such a large interferometer is difficult to beintegrated on a PIC chip. The OFD detector in FIG. 14 can obtain thesame frequency resolution with only few mm delay imbalance and thereforeenable the direct integration on a PIC chip. Alternatively, the obtainedfrequency information may also be used to control the tunable laser forgenerating ultra-linear frequency modulation.

FIG. 15 is an example of a frequency domain OCT integrated with atunable laser, a pair of balanced photodetectors, and a sine/cosineoptical frequency detector integrated on a PIC chip based on thedisclosure of this patent document. In such a frequency domain OCTsystem, the precisely known frequency increment is needed for obtainingthe accurate distance information of the object. An OCT system may beimplemented using an optical interferometer (e.g., a Mach-Zehnderinterferometer in FIG. 1 or Michelson interferometer) with a large delayimbalance to generate f-clock representing the frequency increment fortriggering the data acquisition. Such a large interferometer isdifficult to be integrated on a PIC chip. Alternatively, the obtainedfrequency information may also be used to control the tunable laser forgenerating ultra-linear frequency modulation.

FIG. 16 is an example of an on-chip fiber Bragg grating (FBG)interrogator by integrating a broad band light source, a wavelengthdivision multiplexer (WDM), a N×1 switch, and a sine/cosine opticalfrequency detector integrated on a PIC chip based on the disclosure ofthis patent document. Probe light from a broadband light source isinjected into the fiber with different FBGs at different opticalwavelengths and different locations in the fiber to reflect light ofdifferent wavelengths. A wavelength division multiplexing (WDM) deviceis provided to receive the reflected light and to separate the receivedlight into different optical signals at the different wavelengths asdifferent WDM channels. An optical N×1 switch is used to switch thelight of each WDM channel to the OFD. The wavelength shift of thereflected light at a particular WDM channel caused by the temperature orstrain in the FBG can be precisely detected by the OFD and thewavelength shifts at different WDM channels can be measuredsequentially.

FIG. 17 is an example of an on-chip FBG interrogator by integrating abroad band light source, a wavelength division multiplexer (WDM), and Nsine/cosine optical frequency detectors based on the disclosure of thispatent document. Different WDM channels separated by the WDM device aredirected in different optical waveguides to different sine/cosineoptical frequency detectors, respectively, with one WDM channel per WDMdevice. The wavelength shift of the reflected light caused by thetemperature or strain in each FBG can be precisely detected by thecorresponding OFD. In some implementations, the WDM can be made with anarrayed waveguide grating (AWG). The wavelength shifts at different WDMchannels can be measured simultaneously by the different sine/cosineoptical frequency detectors.

FIG. 18 incudes FIGS. 18A and 18B showing an example of a PICinterrogator chip for interferometric distributed sensing based on anyof the sine/cosine optical frequency detectors disclosed above on a PICchip and a 90° hybrid coherent receiver. FIG. 18B shows that the PBSRsection in FIG. 18A can be replaced with a 2D vertical grating forachieving polarization splitting.

FIG. 19 is an example a PIC interrogator chip for interferometricdistributed sensing based on any of the OFD's disclosed above and a 2×4MMI coupler.

FIG. 20 is an example a PIC interrogator chip for interferometricdistributed sensing based on any of the OFD's disclosed above and a 2×3MMI coupler based on the basic structure in FIG. 18 while using acoherent receiver based a 2×3 MMI coupler to replace the 90° hybridcoherent receiver in FIG. 18A. The two outputs of the returned lightfrom the PB SR are made to interfere with the reference light from C2 toproduce two pairs of interference signals. Similar to FIG. 18B, the PBSRsection can be replaced with a 2D vertical grating coupler to separatethe two orthogonal polarization components.

FIG. 21 Illustrates an example of a PIC interrogator chip disclosed inFIG. 18 to FIG. 20 for different distributed sensing applications,including an optical coherent tomography (OCT) device, optical coherencedomain reflectometer (OFDR), and frequency modulated continuous wave(FMCW) LiDAR. As shown in the left and right hand sides of the PIC chip,for different applications, the lasers at the input of the chip and theoptics at the output of the chip are different. The box on the leftshows the requirements of the laser parameters for differentapplications, while the drawings on the right showing the optics forOFDR, OCT, and LiDAR applications. Because the same chip can be used forthree different sensing applications, the development cost can be sharedand the market size for the chip can be significantly increased.

FIG. 22 illustrates one example of an embodiment of a coherentinterrogator chip with polarization diversity detection for distributedoptical sensing applications, including optical frequency domainreflectometer (OFDR), optical coherence tomography (OCT) and coherentLidar.

FIG. 23 illustrates another example of an embodiment of a coherentinterrogator chip with polarization diversity detection for distributedoptical sensing applications, including optical frequency domainreflectometer (OFDR), optical coherence tomography (OCT) and coherentLidar, in which a free-space micro-circulator is used for reducing theoptical insertion loss of the system.

DETAILED DESCRIPTION

Measuring optical frequencies of light can be performed by an opticalspectrum analysis in various ways, including, for example, 1) using aspatially dispersive element, such as a diffractive grating, tospatially separate different optical frequency components, 2) using atunable narrow band filter, such as a Fabry-Perot resonator or a tunablefiber Bragg grating, to sequentially tune the bandpass frequency toselect different frequency components out of the input light, and 3)performing fast Fourier transfer (FFT) on the output of a Michaelson orMach-Zehnder interferometer as the path difference between the twointerfering arms is varying. The spectral resolution, spectral range,and measurement speed generally counter play with one another, andtherefore it can be challenging to simultaneously achieve goodperformances of all three parameters in certain implementations ofoptical spectrum measurement devices. For example, the resolution andmeasurement range of a Fabry-Perot filter based spectrum analyzer areinversely proportional to each other. Implementations of such techniquesfor achieving good resolutions tend to compromise the measurement rangewhich is set by the free spectral range (FSR) of a device.

For fast scanning tunable laser sources with a desired scanning range(e.g., 160 nm) and a suitable scanning repetition rate (e.g., tens ofkHz), it is desirable to measure the wavelength as a function of time asthe laser wavelength is scanned. Such time-dependent measurements of thelaser wavelength by using above mentioned techniques can be difficult.The technology disclosed in this patent document can be implemented toprovide improved and effective ways for preforming such measurements.

Various devices can be used for obtaining the optical spectruminformation based on polarization analysis of a light signal passingthrough a differential group delay (DGD) element that causes a delaybetween the two orthogonal polarizations. Such devices can be designedto overcome the short comings of some other optical spectrum analyzers,and simultaneously achieve desired high spectral resolution, widespectra range, and high speed. Examples of polarization-based opticalspectrum analyzers include the examples in U.S. Pat. No. 8,345,238 byYao for “Measuring optical spectral property of light based onpolarization analysis”. U.S. Pat. No. 10,895,477 to Yao et. al for“Sine-cosine optical frequency encoder devices based on opticalpolarization properties” by Yao (patent application Ser. No. 15/975,757and U.S. Patent Publication No. US20180372517A1) provides examples ofdevices and techniques for detecting the optical frequency by analyzingoptical power after the light passing through differential group delay(DGD) elements and polarizers. Those patents are incorporated byreference as part of the disclosure of this patent document.

This patent document includes examples of optical frequency detectiondevices suitable to be fabricated in photonics integrated circuits(PICs). The disclosed devices can be made without or minimize bulkoptical components for a wide range of applications and to enable lowcost construction and easy signal processing. In particular, the finalfrequency information can be deducted into a pair of sine and cosinefunctions, similar to that of a commonly used sine/cosine encoder forobtaining the angular or position information in motion controlapplications. Since sine/cosine encoders are widely used in the industryand the interpolation and applications of the signals are well known,such optical frequency detectors can be built by using the disclosedtechnology to achieve low cost and compact size for wide applicationsincluding, e.g., laser frequency measurement and control, FBGinterrogation, and swept-frequency or chirped frequency sensor systems,as will be discussed below.

FIG. 1 a illustrates a prior art example for a k-clock (or f-clock)generator based on Mach-Zehnder interferometer (MZI) commonly used inOCT, OFDR and coherent Lidar systems. The output photocurrents I₁ and I₂from the interferometer's photodetectors (PD) can be expressed as

I ₁ /I ₁₀=1+cos(2πf/FSR+φ ₀),  (1a)

I ₂ /I ₂₀=1−cos(2πf/FSR+φ ₀),  (1b)

where I₁₀ and I₂₀ are amplitudes of the detected photocurrent, which areproportional to the received optical power, the responsibilities of thePD's, and the access losses of the interferometer. The interferometerfree spectral range FSR is defined as:

FSR=1/τ=c/ΔL,  (2)

where τ and ΔL are the delay between the two arms of the interferometerexpressed in time and in optical path length, respectively. SubtractingEq. (2) from Eq. (1) yields

I ₁ /I ₁₀ −I ₂ /I ₂₀=2 cos(2πf/FSR+φ ₀)  (3)

FIG. 1 b illustrates the relationship between measurement range andfrequency measurement resolution. In principle, for the unambiguousdetection of frequency increment, the detection range is limited to halfof the free spectral range (FSR). However, a larger FSR results in alower frequency slope and therefore worse measurement resolution.Conversely, a better frequency measurement resolution requires small FSRor a large delay imbalance between the two arms. For detecting aunidirectional optical frequency sweep, one may simply get zero-crossingfrequencies which have equal frequency spacing for certain applications.

The k-clock generator can be used to generate trigger pulses with equalfrequency spacing for data acquisition so that the data points obtainedare labeled with equally-spaced frequency markers. As shown in FIG. 1 b, such frequency markers can be generated with an imbalancedMach-Zehnder interferometer (MZI) at the zero-crossing points if thetunable laser's frequency variation is unidirectional. Fast FourierTransform (FFT) of the data can be used to obtain the correct distanceinformation for the backscattered signals. One major issue with thisapproach is that a very large delay imbalance between two interferingarms is generally required, especially for applications in which verylarge measurement range is required, such as OFDR and coherent LiDAR.For example, for an OFDR with an intended measurement range of 1 km, adelay imbalance of 2 km may be required. Such a large delay is difficultto be integrated on a PIC chip. In addition, phase noise effect of thelaser will be profound with such a long delay, causing k-clockinaccuracies. In particular, the effect of the laser phase noise mayreverse the direction of laser frequency tuning locally, causing largeerrors in obtained distance information from the FFT.

FIG. 2 shows a 1^(st) embodiment of the disclosed technology in thispatent document in which a 1^(st) and a 2^(nd) Mach-Zehnderinterferometers are formed on a photonic chip by optical waveguidesformed on the chip. The 1^(st) interferometer has a delay imbalance ofΔL between the two interfering optical arms, with the two outputsfollowing Eqs (1) and (3). Two photodetectors PD1 and PD2 are providedto detector the two outputs.

The 2^(nd) interferometer in FIG. 2 has a delay imbalance of ΔL±λ₀/4,where λ₀ is the center wavelength of the laser to be measured. Thisextra delay imbalance of ±λ₀/4 introduces a phase of ±π/2. Consequently,the two outputs from the 2^(nd) interferometer are

I ₃ /I ₃₀=1+sin(2πf/FSR+φ ₀),  (4a)

I ₄ /I ₄₀=1−sin(2πf/FSR+φ ₀),  (4b)

I ₃ /I ₃₀ −I ₄ /I ₄₀=2 sin(2πf/FSR+φ ₀),  (4c)

where I₃ and I₄ are the photocurrent from two photodetectors PD 3 andPD4 of the 2nd interferometer, and I₃₀ and I₄₀ are the amplitudes of thedetected photocurrents, which are proportional to the received opticalpower, the responsibilities of the PD's, and the access losses of theinterferometer. The Eqs. (3) and (4c) can be used to obtain thefollowing two equations

cos(θ+φ₀)=(I ₁ /I ₁₀ −I ₂ /I ₂₀)/2  (5a)

sin(θ+φ₀)=(I ₃ /I ₃₀ −I ₄ /I ₄₀)/2  (5b)

where θ=2πf/FSR  (5c)

By using the well-known sine/cosine interpolation algorithms, both thedirection and the amplitude of θ variations can be obtained, even when θchanges over a large range of multiple of 2π. In particular, θ can beobtained by

$\begin{matrix}{\theta = {{\tan^{- 1}\left( \frac{{I_{3}/I_{30}} - {I_{4}/I_{40}}}{{I_{1}/I_{10}} - {I_{2}/I_{20}}} \right)} - \varphi_{0}}} & (6)\end{matrix}$

when θ changes more than 2π, the fringe counting can be implemented tounwrap the phase since the direction of θ can also be determined, aswill be discussed later. The frequency variation can be readily obtainedas:

$\begin{matrix}{{\Delta f} = {{\frac{1}{2\pi}{\Delta\theta}*{FSR}} = {\frac{1}{2\pi}{\Delta\theta}/\tau}}} & (7)\end{matrix}$

Notably, this technique utilizes both the sine and cosine terms toachieve a very high resolution of frequency variations with an infinitemeasurement range. However, if only one of the sine and cosine terms isavailable, both the frequency resolution and the measurement range arelimited. For example, for a LiDAR to have a measurement range of 300 min air or an OFDR system to have a measurement range of 200 m in fiber,corresponding to a round the trip optical path length of 600 m, thedelay imbalance of the Mach-Zehnder interferometer for generating thef-clock via zero-crossing should be at least 400 m in fiber (600 m inair). The corresponding frequency resolution δf is

$\begin{matrix}{{\delta f} = {\frac{FSR}{2} = {\frac{c}{2*600} = {250{kHz}}}}} & (8)\end{matrix}$

When both the sine detection and cosine detection are used, a minimumdelay imbalance of only 0.92 mm in air is needed to achieve 250 kHzfrequency resolution, assuming 16 bit digital resolution for processingthe data. The estimation above can be obtained by enlarging the FSR inEq. (8) by 2¹⁶ times because 2¹⁶ data points can be taken in half theperiod shown in FIG. 1 b . In practice, a delay imbalance can bemultiple of the minimum delay imbalance for better frequency resolutionbecause the sine/cosine interpretation algorithm enabled by the designof FIG. 2 is capable of unwrapping the phase beyond 2π. For example, ifthe interferometer is made with silicon waveguides with an effectiveindex of 3, for a delay imbalance of 9 mm, the waveguide path lengthdifference is only 3 mm, which can be easily fabricated in siliconphotonics platform. Note that the larger the delay imbalance, the higherthe data acquisition and processing rate is required, because theinterferometer converts the optical frequency variation to optical powervariation at a higher rate. In other words, the power variation ratefrom the interferometer in response to the frequency variation isproportional to the delay imbalance of the interferometer.

The OFD device in FIG. 2 can be constructed using waveguides andwaveguide components on a substrate such as multi-mode interferometer(MMI) devices and optical couplers to provide desired optical couplingamongst the coupled optical waveguides for the interferometers. Theentire device is an integrated on-chip system as part of a photonicsintegrated circuit (PIC) over a substrate and can be made compact inlarge scales at a low cost. The OFD device in FIG. 2 and other devicesdescribed in this patent document may be alternatively implemented usingfiber segments and fiber couplers. In each of the two waveguideinterferometers, at least one of the two optical arms of theinterferometer is coupled to a tuning device that can change or tune theoptical length of an optical arm. For example, optical heaters arecoupled to the lower waveguide arm in each of the two interferometers inFIG. 2 to control the refractive index or/and length of the heatedwaveguide to control the relative phase delay between the two opticalarms. Other devices other than heaters may be used to control therefractive index or/and length of either or both of the two waveguidesto control the relative phase delay between the two optical arms, e.g.,an electro-optic modulator. In the second interferometer, a phaseshifter device is also coupled to the lower waveguide arm to provide thedesired phase shift.

FIG. 3 shows an example of a polarization insensitive embodiment of thesine/cosine optical frequency detector (OFD). Because the waveguidesused for making the frequency detector are of single mode waveguides,either TE mode or TM mode, for an input light having an arbitrarypolarization, the configuration in FIG. 2 may be problematic. Forexample, if the waveguides in FIG. 2 only support TE mode, the TM modecomponents in the input light may be heavily attenuated, resulting inmeasurement inaccuracies. To overcome the problem, a polarization beamsplitter is implemented in FIG. 3 to first split the input light into aTE waveguide in the upper branch of the optical circuit and a TM modewaveguide in the lower branch of the optical circuit. The light in theTE waveguide of the upper branch then enter into a sine/cosine OFD shownin FIG. 2 . The light in the TM mode waveguide of the lower branch isthen converted to TE mode via a TM-to-TE mode converter before enteringan identical sine/cosine OFD as in the upper branch. This way,regardless the input polarization state, one can always have sufficientoptical power in either the upper or lower branch OFD for obtaining theoptical frequency information. In particular, by combining the signalsin the upper and lower branches, the following OFD signals can beobtained:

cos(θ+φ₀)=[(I ₁ /I ₁₀ −I ₂ /I ₂₀)+(I ₅ /I ₅₀ −I ₆ /I ₆₀)]/4,  (9a)

sin(θ+φ₀)=[(I ₃ /I ₃₀ −I ₄ /I ₄₀)+(I ₇ /I ₇₀ −I ₈ /I ₈₀)]/4,  (9b)

Therefore, the OFD output in FIG. 3 is insensitive to the inputpolarization variations. If the input is of pure TE, I₅=I₆=I₇=I₈=0, thefirst terms in the right hand side of the Eqs. (9a) and (9b) contributein the calculation. If the input is of pure TM, I₁=I₂=I₃=I₄=0, thesecond terms in the right hand side of the Eqs. (9a) and (9b) contributein the calculation. When the input has both TE and TM components, boththe first and second terms contribute in the calculation.

FIG. 4 shows one example of an analog circuit to implement an algorithmfor calculating the instant phase or frequency of the light signal. Suchan algorithm can also be implemented by perform the digital mathematicalcalculations shown in FIG. 4 to first obtain θ variation Δθ and then theoptical frequency variation Δf using Eq. (7). Such an algorithm can alsobe implemented by perform the digital mathematical calculations shown inFIG. 5 to first obtain θ variation Δθ and then the optical frequencyvariation Δf using Eq. (7).

FIG. 5 shows a digital circuit for obtaining for obtaining the instantphase or frequency of the light signal. The comparators in FIG. 5 a areto get the zero crossing positions for period counting, as shown in thetop and middle graphs in FIG. 5 b . The ADC in FIG. 5 a is to

${\theta = {\tan^{- 1}\left( \frac{\sin Y2}{\cos Y1} \right)}},$

digitize the data and then the DSP is used to calculate the instantphase using as shown in the bottom graph in FIG. 5 b , where Y1 and Y2are quantities at the right hand side of Eqs. (5a) and (5b),respectively, if the embodiment of FIG. 2 is used. Alternatively, Y1 andY2 are quantities at the right hand side of Eqs. (9a) and (9b),respectively, if the embodiment of FIG. 3 is used.

FIG. 6 shows an example of another embodiment of a sine/cosine opticalfrequency detector (OFD) based on the technology disclosed in thispatent document, which combines a Mach-Zehnder interferometer (MZI) of alarge free spectral range (FSR) with the pair of interferometers of asmall FSR to enable absolute optical frequency detection with a highresolution. Input light is polarized (e.g., in the TE polarization mode)and is split by an optical coupler, such as an input multimodeinterferometer (MMI) coupler into two polarized beams in the samepolarization which are received by two waveguides. The upper branch goesto the OFD described in FIG. 2 and the lower branch goes to another MZIwith a much smaller delay imbalance of δL or a much larger free spectralrange (FSR).

FIG. 7 shows using the combination of an optical frequency detector(OFD) of a large FSR an OFD of a small FSR described in FIG. 6 formaking an absolute frequency detector. The value of the large FSR₂ ofthe lower branch in FIG. 6 can be chosen such that in the frequency (orwavelength) range of interest, the detected optical power change islimited in the first quarter of the cosine function for the coarseabsolute optical frequency measurement, as shown in the figure. Withinthis quarter period, there is a large amount of periods produced by theOFD of the small FSR. So long as the frequency resolution of the OFDwith the large FSR is sufficiently fine to resolve a period of the OFDof the small FSR, the absolute frequency of the light source can beobtained unambiguously. For example, in some implementations, the largeFSR₂ may be set to be on the order of thousands GHz while the small FSR₁may be on the order of GHz.

In some implementations, the MZI with the large FSR in FIG. 6 can bereplaced with a directional coupler having a strong wavelengthdependence similar to that in FIG. 7 , such as a directional couplerbased coarse WDM with one port output 1550 nm signal and the other portoutput 1310 nm signal.

FIG. 8 illustrates such a sine/cosine OFD combining a pair of MZI's of asmall free spectral range (FSR₁=c/ΔL) with a directional coupler toenable absolute optical frequency detection. Input light is polarized(e.g., in the TE polarization mode) and is split by an input MMI intotwo polarized beams in the same polarization that are directed into twowaveguides. The upper branch goes to the OFD described in FIG. 2 withoptical detectors PD1, PD2, PD3 and PD4 while the lower branch goes to acoarse WDM made with a directional coupler with a large free-spectralrange (FSR₂). The directional coupler can be structured to include twooptical waveguides that are placed adjacent to each other to causeoptical evanescent coupling between the two optical waveguides to splitthe input light into two optical signals to be directed by two opticaldetectors PD5 and PD6. This directional coupler may be structured toexhibit a large FSR₂ and with two output ports for exporting opticaloutput signals that are generated by splitting the input to thedirectional coupler.

The coupling ratios of the two output ports of the directional couplercan be used to measure the optical frequency f of the light based on theoutputs of the two optical detectors PD5 and PD6, and the FSR₂:

$\begin{matrix}{\frac{I_{5}}{I_{50}} = {1 + {\cos\left( {{\frac{2\pi}{{FSR}_{2}}f} + \varphi_{0}} \right)}}} & \left( {10a} \right) \\{\frac{I_{6}}{I_{60}} = {1 - {\cos\left( {{\frac{2\pi}{{FSR}_{2}}f} + \varphi_{0}} \right)}}} & \left( {10b} \right) \\{{f(t)} = {{\left( \frac{{FSR}_{2}}{2\pi} \right){acos}} - {\frac{1}{2}\left\lbrack {\frac{I_{5}}{I_{50}} - \frac{I_{6}}{I_{60}}} \right\rbrack}}} & \left( {10c} \right)\end{matrix}$

FIG. 9A illustrates an absolute sine/cosine OFD made with a differentkind of interferometer including a 1×2 coupler (coupler 1) and a 2×3 MMIcoupler with a small free spectral range (FSR₁=c/ΔL which may be on theorder of GHz in some implementations). This interferometer is used toreplace the pair of MZI's in FIG. 2 and FIG. 6 . The optical coupler 1splits the polarized input light (e.g., in the TE polarization) into afirst optical signal in an upper optical branch with an optical coupler2 and a second optical signal in the lower optical branch. Theinterferometer in the upper branch is structured to generate threeoptical output interferometer signals in different phases relative toone another, e.g., with phases in 0 degree, 120 degrees and −120 degreesas shown in the example illustrated in FIG. 9A. The interferometer inthe upper branch with a relatively small FSR₁ (e.g., on the order ofGHz) can be implemented to use a 2×3 MMI coupler to combine the twodifferent optical signals in two optical waveguides that are generatedby the beam splitting at the optical coupler 2 to cause opticalinterference of those two optical signals and to produce the threeoptical output interferometer signals in different phases relative toone another. Optical detectors PD1, PD2 and PD3 are provided to detectthe three optical output interferometer signals. This OFD can becombined with an OFD of a FSR₂=c/δL in the lower branch produced by theoptical coupler 1. The FSR₂ is much larger than FSR₁ to achieve absolutefrequency detection, similar in principle to that of FIG. 7 . Forexample, FSR₂ may be on the order of thousands GHz while the small FSR₁may be on the order of GHz in some implementations. In the example shownin FIG. 9A, the signals from the three output ports of the 2×3 MMIcoupler in different phase values can be expressed as:

$\begin{matrix}{I_{1} = {C + {B\cos{\theta_{1}(t)}}}} & \left( {11a} \right) \\{I_{2} = {C + {B{\cos\left\lbrack {{\theta_{1}(t)} + {120{^\circ}}} \right\rbrack}}}} & \left( {11b} \right) \\{I_{3} = {C + {B{\cos\left\lbrack {{\theta_{1}(t)} - {120{^\circ}}} \right\rbrack}}}} & \left( {11c} \right) \\{{\theta_{1}(t)} = {\frac{2\pi}{{FSR}_{1}}{f(t)}}} & \left( {11d} \right)\end{matrix}$

FIG. 9B shows the phase relationship of the 3 outputs from the 2×3 MMIcoupler in FIG. 9A, which are nominally 120° out of phase with eachother. Based on the above relationships, the absolute value of theoptical frequency f can be determined from the three signals:

$\begin{matrix}{{\sin{\theta_{1}(t)}} = {\left( {I_{2} - I_{1}} \right)\left( {\sqrt{3}B} \right)}} & \left( {12a} \right) \\{{\cos{\theta_{1}(t)}} = {\left\lbrack {{2I_{1}} - \left( {I_{2} + I_{3}} \right)} \right\rbrack/\left( {3B} \right)}} & \left( {12b} \right) \\{{\tan{\theta_{1}(t)}} = {\sqrt{3}\left( {I_{2} - I_{1}} \right){/\left\lbrack {{2I_{21}} - \left( {I_{2} + I_{3}} \right)} \right\rbrack}}} & \left( {12c} \right) \\{{f(t)} = {\frac{{FSR}_{1}}{2\pi}{atan}\left\{ \frac{\sqrt{3}\left( {I_{2} - I_{1}} \right)}{\left\lbrack {{2I_{1}} - \left( {I_{2} + I_{3}} \right)} \right\rbrack} \right\}}} & \left( {12d} \right)\end{matrix}$

Phase unwrapping algorithms can be used to account phase changes beyond2π in Eq. (12) to enable a large frequency measurement range. Theincremental optical frequency f of the light source can therefore beobtained.

FIG. 9C shows an alternative configuration in which the OFD with thelarge FSR (FSR₂) in the lower optical branch at the output of opticalcoupler 1 can be replaced with a directional coupler based WDM to enableabsolute optical frequency detection, similar to the configuration ofFIG. 8 .

FIG. 10 illustrates another sine/cosine OFD similar to that of FIG. 9A,except that the optical interferometer in the upper optical branchproduced by the optical coupler 1 replaces 2×3 MMI coupler in FIG. 9Awith a 2×4 MMI coupler to produce four optical interference signals with4 different phase values. The upper optical branch interferometer isdesigned to produce two different output interference signals to firstand second photodetectors PD1 and PD2 as cosine functions of the opticalfrequency and two other different interference signals to third andfourth photodetectors PD3 and PD4 as sine functions of the opticalfrequency:

I ₁ /I ₁₀=1+cos(2πf/FSR ₁)

I ₂ /I ₂₀=1−cos(2πf/FSR ₁)

I ₃ /I ₃₀=1+sin(2πf/FSR ₁)

I ₄ /I ₄₀=1−sin(2πf/FSR ₁)

In FIG. 10 , a 1×2 coupler and the 2×4 MIMI coupler are used in theupper optical branch to form an interferometer of a small FSR (FSR₁=c/ΔLon the order of GHz). This OFD can be combined with an OFD of a muchlarger FSR (FSR₂=c/δL on the order of thousands GHz) for absolutefrequency detection, similar in principle to that of FIG. 7 .Alternatively, the OFD with the large FSR can be replaced with adirectional coupler based WDM to enable absolute optical frequencydetection, similar to the configuration of FIG. 8 and FIG. 9 . The fouroutputs from the 2×4 MMI coupler can be expressed as:

$\begin{matrix}{\frac{I_{1}}{I_{10}} = {1 + {\cos{\theta_{1}(t)}}}} & \left( {13a} \right) \\{\frac{I_{2}}{I_{20}} = {1 - {\cos{\theta_{1}(t)}}}} & \left( {13b} \right) \\{\frac{I_{3}}{I_{30}} = {1 + {\sin{\theta_{1}(t)}}}} & \left( {13c} \right) \\{\frac{I_{4}}{I_{40}} = {1 - {\sin{\theta_{1}(t)}}}} & \left( {13d} \right) \\{{f(t)} = {\frac{{FSR}_{1}}{2\pi}{{atan}\left\lbrack {\left( {\frac{I_{3}}{I_{30}} - \frac{I_{3}}{I_{30}}} \right)/\left( {\frac{I_{1}}{I_{10}} - \frac{I_{2}}{I_{20}}} \right)} \right\rbrack}}} & \left( {13e} \right)\end{matrix}$

The incremental optical frequency can be obtained from Eq. (13) with ahigh resolution and the absolute optical frequency can be obtained withthe OFD with much smaller FSR.

FIG. 11 shows an example of a sine/cosine OFD system for minimizingpolarization sensitivity of the sine/cosine OFD's described above. Ingeneral SiO₂ based photonics integrated circuits (PICs) are notpolarization sensitive, while Si and SiN based PICs may be polarizationsensitive. In FIG. 11 , the incoming lightwave with an arbitrarypolarization is split by a polarization splitter and rotator (PB SR)with the TM polarization rotated into the TE polarization so that thetwo output optical signals from the PBSR are in the same polarization,e.g., in the TE polarization as shown. Two identical OFD's described inFIGS. 8-10 are included to simultaneously detect the optical frequencyin the two arms to eliminate the polarization sensitivity, similar tothe case of FIG. 3 .

FIG. 12 shows an example of a tunable laser integrated with any of thesine/cosine OFD disclosed above. The detected frequency b thesine/cosine OFD can be fed back to control the laser frequency to enablethe device in FIG. 12 to function as an optical frequency synthesizer.As illustrated, a tunable laser is provided as the light source togenerate a laser beam at a laser frequency and the laser beam from thetunable laser is directed to an optical coupler or splitter such as amulti-mode interferometer (MMI) device as shown to be split into a laseroutput beam and a laser frequency monitor beam. A first opticalwaveguide or fiber is provided and coupled to the optical coupler toreceive and direct the laser output beam as the output beam and a secondoptical waveguide or fiber is provided and coupled to the opticalcoupler to receive and direct the laser frequency monitor beam to thesine/cosine OFD. A laser feedback control circuit is provided to use themeasured laser frequency from the sine/cosine OFD to generate a feedbackcontrol signal for laser frequency control and to apply the laserfeedback control signal to the tunable laser to tune the laser frequencyin various ways. For example, this device may be used to achieve thefollowing functionalities: 1) stabilize the laser output to any fixedfrequency in the range; 2) generate a frequency ramp with a highlinearity; 3) generate an arbitrary frequency variation waveform.

FIG. 13 illustrates an example of an optical coherence domainreflectometer (OFDR) integrated with a tunable laser, a pair of balancedphotodetectors (PD5 and PD6 with an optional variable optical attenuator(VOA) in front of at least one photodetector), and a sine/cosine OFDbased on the disclosure of this patent document. In such a system, theprecisely known frequency increment is used for obtaining the accuratedistance information of the object. An OFDR generally uses an opticalinterferometer (either of Mach-Zehnder or Michelson) with a large delayimbalance shown in FIG. 1 , on the order of 100 meters, to generatef-clock representing the frequency increment for triggering the dataacquisition. Such a large interferometer can be difficult to beintegrated on a PIC chip. In addition, it may also suffer polarizationfading caused by the polarization variations in the long fiber delay.Here the OFD detector can obtain the same frequency resolution with onlyfew mm delay imbalance and therefore enable the direct integration,without the problem of polarization fading. Alternatively, the obtainedfrequency information may also be used to control the tunable laser forgenerating ultra-linear frequency modulation.

More specifically, the example OFDR system in FIG. 13 uses a tunablelaser as a light source on a photonics integrated circuit (PIC) chip toproduce an optical beam for optical sensing. A first beam splitter,which may be a MMI, is formed on the PIC chip and coupled to receive theoptical beam from the tunable laser to split the optical beam into afirst optical beam and a second optical beam. A second beam splitter,which may be another MIMI, is formed on the PIC chip and coupled toreceive at least a portion of the first optical beam to split thereceived portion of the first optical beam into a probe beam and areference beam. An optical port is formed as shown on the upper rightcorner of the PIC chip and is coupled to receive the probe beam from thesecond beam splitter to direct the probe beam into an optical fiber tocause light reflections or scattering inside the fiber at differentlocations to generate returned probe light which propagates back to theoptical port and to the second beam splitter. The returned probe lightcarries information of temperature, strain, stress at these locations inthe optical fiber and is used to extract such information. An opticalreflector is provided either on or near the PIC chip to receive thereference beam from the second beam splitter and to reflect thereference beam back to the second beam splitter to interfere with thereturned probe light from the fiber at the second beam splitter toproduce an interference signal.

An optical detector module is coupled to receive the interference signalgenerated by the second optical beam splitter MIMI. In the example inFIG. 13 , this optical detector module includes a photodetector PD5coupled directly to receive a portion of the interference signalgenerated at the second MMI and another optical detector PD6 to alsoreceive a portion of the interference signal generated at the secondMMI. A beam coupler is formed between the first and second MMI beamsplitters so that a portion of the interference signal generated at thesecond MMI is split into the PD6.

Downstream from the first optical beam splitter MIMI, an opticalfrequency detection device is provided on the PIC chip and is located toreceive the second optical beam from the first optical beam splitter MMIto detect and measure a frequency variation in the second optical beamcaused by a frequency variation in light of the optical beam produced bythe tunable laser. This optical frequency detection device can beimplemented in various configurations as disclosed in this patentdocument. As illustrated, the digital circuit has a signal input portthat is coupled to the photodetectors Pd5 and PD6 and another signalinput port that is coupled to receive the information on the measuredlaser frequency of the laser. In operation, the measured frequencyvariation in the second optical beam by the optical frequency detectiondevice is used to compute a fast Fourier transform of the interferencesignal to obtain information contained in the returned probe light ontemperature, strain, or stress as a function of a location in the fiber.In this design, the measured frequency variation in the second opticalbeam is used to generate f-clock representing the frequency incrementfor triggering the data acquisition by the digital circuit based on thedetector outputs from PD5 and PD6. In addition, an electronic controlmodule can be used to, based on the measured frequency variation in thesecond optical beam, to provide a feedback control to control or adjustthe frequency of the tunable laser.

FIG. 14 illustrates an example of a chirped LiDAR integrated with atunable laser, a pair of balanced photodetectors (PD5 and PD6 with anoptional variable optical attenuator (VOA) in front of at least onephotodetector), and a sine/cosine OFD based on the disclosure of thispatent document. The tunable laser may be implemented by a frequencymodulated continuous wave (FMCW) laser under control by a feedbackcontrol signal from the control electronics of the chirped lidar. Insuch a system, the precisely known frequency increment is essential forobtaining the accurate distance information of the object. A Lidargenerally uses an optical interferometer (either of Mach-Zenhder orMichelson) with a large delay imbalance shown in FIG. 1 , on the orderof 100 meters, to generate f-clock representing the frequency incrementfor triggering the data acquisition. Such a large interferometer isimpossible to be integrated on a PIC chip. In addition, it may alsosuffer polarization fading caused by the polarization variations in thelong fiber delay. Here the OFD detector can obtain the same frequencyresolution with only few mm delay imbalance and therefore enable thedirect integration, without the problem of polarization fading.Alternatively, the obtained frequency information may also be used tocontrol the tunable laser for generating ultra-linear frequencymodulation.

FIG. 15 shows an example of a frequency domain optical coherenttomography (OCT) device integrated with a tunable laser, a pair ofbalanced photodetectors, and a sine/cosine OFD based on the disclosureof this patent document. Such an OCT device can be used for medicalimaging and various other sensing applications. In such a system, theprecisely known frequency increment is essential for obtaining theaccurate distance information of the object. An OCT system generallyuses an optical interferometer (either of Mach-Zehnder or Michelson)with a relatively large delay imbalance, to generate f-clockrepresenting the frequency increment for triggering the dataacquisition. Such a large interferometer is difficult to be integratedon a PIC chip. Here the OFD detector can obtain the same frequencyresolution with only sub-millimeter delay imbalance and therefore enablethe direct integration. Alternatively, the obtained frequencyinformation may also be used to control the tunable laser for generatingultra-linear frequency modulation.

FIG. 16 shows an example of an on-chip fiber Bragg grating (FBG)interrogator by integrating a broad band light source, a wavelengthdivision multiplexer (WDM), a N×1 switch, and a sine/cosine OFD based onthe disclosure of this patent document. A series of fiber Bragg grating(FBG) segments are coupled to or formed within, a sensing fiber atdifferent locations and at different resonant wavelengths. The sensingfiber with FBG sensors can be deployed into various devices, systems, orstructures for obtaining useful measurements by detecting returnedoptical signals from the FBGs and processing the detected signals. Thesensing fiber can be packaged in a strong, rugged material or package towithstand harsh environmental conditions and the presence multiple FBGsensors allow simultaneous measurements at different locations. Thebroadband light source is injected into the fiber with N FBG's reflectdifferent wavelengths towards the WDM for separating them into Nwaveguides. The N×1 switch is used to switch the light of each WDMchannel to the OFD. The wavelength shift of the reflected light causedby the temperature or strain in the FBG can be precisely detected by theOFD. The WDM can be made with arrayed waveguide grating (AWG). Note thatthe spacing between the two adjacent wavelength channels should belarger than the wavelength shift of each FBG caused by physicalparameters to be sensed, such as the temperature, the strain, or thevibration.

FIG. 17 shows an example of an on-chip FBG interrogator by integrating abroad band light source, a wavelength division multiplexer (WDM) with Nwavelength channels, and N sine/cosine OFDs based on the disclosure ofthis patent document. The broadband light source is injected into thefiber with N FBG's reflect different wavelengths towards the WDM forseparating them into N waveguides. Each waveguide is connected to anOFD. The wavelength shift of the reflected light caused by thetemperature or strain in each FBG can be precisely detected by thecorresponding OFD. The WDM can be made with arrayed waveguide grating(AWG). The embodiment of FIG. 17 can simultaneously detect N FBGchannels, while the embodiment of FIG. 16 can sequentially detect N FBGchannels. Note that the spacing between the two adjacent wavelengthchannels should be larger than the wavelength shift of each FBG causedby physical parameters to be sensed, such as the temperature, thestrain, or the vibration.

In applications, the disclosed OFD devices in this patent document canbe used to perform distributed sensing in an optical fiber.

FIG. 18A discloses an example of a first PIC interrogator chip designfor interferometric distributed sensing based on an OFD disclosed aboveand a 90° hybrid coherent receiver to implement two 90-degree hybridcoherent receivers shown in two dashed boxes to form a balanceddetection amplification scheme. This chip example includes a laser inputport to receive input laser light and an output port that is coupled todirect probe light either into an optical fiber (for OFDR applications)or on to objects in free-space (for OCT or LiDar applications) to besensed and to collect returned probe light from the fiber or theobjects. Therefore, the optical output port shown in FIG. 18A serves asan optical input port for the PIC interrogator chip to receive thereturned probe light. For the case of the probe light into the opticalfiber, the Rayleigh back scattering which contains the local informationof temperature, strain, or stress in the fiber, along with the locationinformation, returns to the PIC interrogator chip to be analyzed. Forthe case of the probe light onto the objects in free space, thereflected or backscattered light from the objects containing thelocation, strength of reflection, speed, and retardation information ofthe objects collected and directed to the PIC interrogator to beanalyzed. A small portion of the laser input (e.g., 5-10%) is firstsplit by optical coupler C1 into a sine/cosine optical frequencydetector to detect its instant frequency to be used as a k-clock orfrequency clock for the system. The remaining portion of the laser inputis further split out a small portion (e.g., 5-10%) by a second opticalcoupler C2 as a reference via a third optical coupler C3 to enter afirst 90° hybrid coherent receiver (with photo detectors PD1, PD2, PD3and PD4) and a second 90° hybrid coherent receiver (with photo detectorsPD5, PD6, PD7 and PD8) after going through an optical delay betweenoptical couplers C2 and C3. This delay is useful for OCT operation tobalance out the optical path length of the returned light (reflected orbackscattered) from the sample only if the coherence length of the laserlight is shorter than the optical path length of the returned light asfor the case of OCT operation. The light from the optical coupler C2 issplit by the optical coupler C2 into two waveguides as two opticalreference signals or optical local oscillator signals LO1 and LO2 to thefirst and second 90° hybrid coherent receivers, respectively. Downstreamfrom the second optical coupler C2 in the main optical path between thelaser input port and the optical output port of the PIC interrogatorchip, the remainder of the input laser light is used as the probe laserlight for sensing is the output at the optical output port which alsoserves as an optical input port for receiving the returned probe light.

As shown in FIG. 18A, an optional semiconductor optical amplifier (SOA)may be placed in the main optical path between the second opticalcoupler C2 and the output port the PIC interrogator chip to boost oramplify the optical power of the probe light power exiting from thechip. The output probe light is directed to a sample for opticalsensing. In operation, the returned light from the sample illuminated bythe output probe light may contain light in an arbitrary state ofpolarization in general and a portion of the returned light from thesample (e.g., 50%) is split out by an optical coupler C4 for opticaldetection. This portion of the returned light split out by the opticalcoupler C4 is directed to enter a polarization beam splitter and rotator(PBSR) which separates the returned light from the optical coupler C4into TE and TM optical beams and then rotates the polarization of one ofthe beams so the two beams are in the same polarization, e.g., rotatingthe polarization of the TM optical beam to change it into a TE opticalbeam. As illustrated, an optional variable optical attenuator (VOA) maybe placed in the output of the PBSR to adjust the signal level. The twooutputs in the same polarization (e.g., TE) from the PBSR are thendirected into the first and second 90° hybrid coherent receivers,respectively and are made to interfere with the reference light fromoptical couplers C2 and C3 to produce two pairs of I-Q interferencesignals V₁, V₂, V₃, and V₄. The optical coupler C3 is placed to receivethe reference light split out by the second optical coupler C2 andsplits the received reference light into two reference light beams forthe two 90-degree hybrid coherent receivers shown in two dashed boxes toform a balanced detection amplification scheme to produce the fouroutputs V₁, V₂, V₃, and V₄. The phase information can be obtained byusing sin φ(t)=(V₁+V₃)/(V₁₀+V₃₀) and cos φ(t)=(V₂+V₄)/(V₂₀+V₄₀).

FIG. 18B shows that the PBSR section in FIG. 18A can be replaced with a2-dimensional (2D) vertical grating coupler (VGC) for achievingpolarization splitting. Light from coupler C2 enters coupler C5 andpropagates in a waveguide supporting TE mode towards the 2D VGC, whichthen either couples into an optical fiber or exits into free space as asensing light beam. The light beam reflected by the sample or objects tobe sensed contains both TE and TM polarization components and thenre-enter the 2D VGC. The 2D VGC separates the TE and TM components ofthe reflected beam into a 1^(st) waveguide supporting TE mode and asecond waveguide supporting the TM modes. The light beam in the 1^(st)waveguide produces a 1^(st) reflected beam. A TM to TE mode converterconverts the TM mode to a TE mode in a TE waveguide to produce a 2^(nd)reflected beam. The first reflected beam enters the 1^(st) 90° hybrid tointerfere with the reference light from C3 and the 2^(nd) reflected beamenters the 2^(nd) 90° hybrid to interfere with another reference lightfrom C3 to produce two pairs of I-Q interference signals. The balanceddetection amplification scheme produces four outputs V₁, V₂, V₃, and V₄.The phase information can be obtained by using sinφ(t)=(V₁+V₃)/(V₁₀+V₃₀) and cos φ(t)=(V₂+V₄)/(V₂₀+V₄₀)

FIG. 19 discloses an example of a second PIC interrogator chip designfor interferometric distributed sensing based on an OFD and a 2×4 MMIcoupler. The operation of the device is almost the same as that of FIG.18 , except that here a coherent receiver based on a 2×4 MMI coupler isused to replace the 90° hybrid coherent receiver in FIG. 18 . The twooutputs of the returned light from the PBSR are made to interfere withthe reference light from C3 to produce two pairs of to produce two pairsof I-Q interference signals. The balanced detection amplification schemeproduces four outputs V₁, V₂, V₃, and V₄. The phase can be obtained byusing sin φ(t)=(V₁+V₃)/(V₁₀+V₃₀) and cos φ(t)=(V₂+V₄)/(V₂₀+V₄₀). Similarto FIG. 18B, the PBSR section can be replaced with a 2D vertical gratingcoupler to separate the two orthogonal polarization components.

FIG. 20 discloses an example of a third PIC interrogator chip design forinterferometric distributed sensing based on an OFD and a 2×3 MMIcoupler. The operation of the device is almost the same as that of FIG.18 , except that here a coherent receiver based on a 2×3 MMI coupler isused to replace the 90° hybrid coherent receiver in FIG. 18 . The twooutputs of the returned light from the PBSR are made to interfere withthe reference light from C2 to produce three of interference signals forobtaining the returned phase information φ(t), which has the sameexpression of Eq. (12), with φ(t) replacing θ₁(t). Similar to FIG. 18B,the PBSR section can be replaced with a 2D vertical grating coupler toseparate the two orthogonal polarization components.

FIG. 21 shows an example for how a PIC interrogator chip can be used fordifferent distributed sensing applications, including OCT, OFDR, FMCWLiDAR and Distributed Acoustic Sensing (DAS). As shown in the left andright hand sides of the PIC chip, for different applications, the lasersat the input of the chip and the optics at the output of the chip aredifferent while using the same PIC interrogator chip. The box on theleft shows the requirements of the laser parameters for differentapplications, while the drawings on the right showing the optics forOFDR, OCT, and LiDAR applications. Because the same PIC interrogatorchip can be used for three different sensing applications, thedevelopment cost can be shared and the market size for the chip can besignificantly increased.

In operation, the phase information of the returned signal can bedemodulated as a function of the optical frequency in the returnedsignal while the laser frequency is tuned with time. The frequencyincrement can be precisely detected with the optical frequency detector.Taking the FFT of the phase of the returned signal, the reflection vs.distance information can be obtained and therefore can be used forachieving distributed sensing. The algorithms for obtaining the requiredsensing information, such as temperature, strain, birefringence,scattering strength, speed and direction of moving objects, etc. are thesame as those of OCT, OFDR, and LiDAR.

FIGS. 22 and 23 show two specific examples of a coherent interrogatorchip with polarization diversity detection for distributed opticalsensing applications, including optical frequency domain reflectometer(OFDR), optical coherence tomography (OCT) and coherent LiDAR based onthe PIC interrogator chip design in FIG. 21 . FIGS. 22 and 23 are notpart of the parent application under U.S. patent application Ser. No.17/515,050.

FIG. 22 illustrates one example of an embodiment of a coherentinterrogator chip with polarization diversity detection for distributedoptical sensing applications, including optical frequency domainreflectometer (OFDR), optical coherence tomography (OCT) and coherentLidar. Light input is first split by coupler C1 into two paths, with oneof them going into the optical frequency detector for measuring theoptical frequency and the other going through a downstream opticalcoupler C2 to be further split into two beams: one as the probe lightgoing out of the chip via the output port for sensing and another one toan on-chip optical coupler C4 as an optical reference signal. The probelight going out of the chip via the output port for sensing may be themajority of the beam after the splitting at coupler C2 (e.g., 95%) andcontinues to go through another optical coupler C3 before sending out asthe sensing beam. The returned sensing signal received at the outputport is split by the optical coupler C3 before going through the PBSR sothat the two orthogonal polarizations are split into two beams of thesame polarization (e.g., TE polarization as illustrated) along twooptical paths after the PB SR. Those two beams of the same polarizationare directed to two optical couplers C5 and C6 for optical detection.Referring back to the optical coupler C2, the weaker light split by theoptical coupler C2 (e.g., 5% of the input light to C2) is directed to anon-chip optical coupler C4 as an optical reference signal and is firstsplit by optical coupler C4 as two optical reference signals in twooptical paths to two optical couplers C5 and C6 to interfere with thereturned sensing signals via couplers C5 and C6 to produce two sets ofbeat signals containing the information of the locations and strengthsof the targets being sensed. At each optical coupler C5 or C6, twooutput interference signals are directed to two photodetectors,respectively: photodetectors PD3 and PD4 are used to respectivelyreceive and detect two interference signals produced at the opticalcoupler C5 and photodetectors PD5 and PD6 are used to respectivelyreceive and detect two interference signals produced at the opticalcoupler C6.

FIG. 23 illustrates another example of an embodiment of a coherentinterrogator chip with polarization diversity detection for distributedoptical sensing applications, including optical frequency domainreflectometer (OFDR), optical coherence tomography (OCT) and coherentLidar, in which a free-space micro-circulator is used for reducing theoptical insertion loss of the system. Different from the input/outputdesign for the coherent interrogator chip in FIG. 22 which uses the sameoptical port on the chip for sending out probe light and for receivingthe returned probe light, the coherent interrogator chip in FIG. 23 hastwo separate optical ports on the chip: (1) a first optical port as thedesignated probe output port for sending out probe light from the chipto a sensing target or region and (2) a second t optical port as thedesignated probe input port for receiving the returned probe light fromthe target or region so that the returned probe light will be directedonly to the designated probe input port and will not be directed to thedesignated probe output port. To achieve this, an optical circulator iscoupled to the two designated optical ports on the chip and has 3 portsmarked as Port 1, Port 2 and Port 3, respectively, to circulate lightfrom Port 1 to Port 2, from Port 2 to Port 3, and from Port 3 to Port 1.With this 3-port optical circulator, the probe light output by the probeoutput port is directed to Port 1 and then to Port 2 to the target andthe returned probe light from the target received by Port 2 is directedto Port 3 which is received by the probe input port. This designeliminates the need for the on-chip optical coupler C3 and thus theoptical loss by the coupler C3 in detecting the returned probe light inthe design in FIG. 22 by providing a waveguide to link the PBSR to theprobe input port to improve the light collection efficiency fordetecting the returned probe light.

The technical features disclosed in this patent document may beimplemented in various ways to construct various devices. Some examplesare provided below.

Item 1. a device for measuring an optical frequency of light,comprising:

-   -   a substrate;    -   first optical waveguides integrated to and supported by the        substrate and coupled to form a first Mach-Zehnder        interferometer having two interfering optical arms and an input        optical port to receive a first portion of input light at an        input optical wavelength that is split into the two interfering        optical arms and an output optical port to receive and combine        light from the two interfering optical arms to produce two first        optical output interferometer signals;|    -   two first photodetectors supported by the substrate and located        to receive the two first optical output interferometer signals,        respectively, wherein the two first photodetectors produce first        and second detector signals, respectively, and each of the first        and second detector signals varies as a sine function of an        optical frequency corresponding to the input optical wavelength;    -   second optical waveguides integrated to and supported by the        substrate and coupled to form a second Mach-Zehnder        interferometer having two interfering optical arms and an input        optical port to receive a second portion of the input light        which is split into the two interfering optical arms, and an        output optical port to receive and combine light from the two        interfering optical arms to produce two second output        interferometer signals, wherein the second Mach-Zehnder        interferometer is structured to have a phase difference between        the two interfering arms different by one quarter of the input        optical wavelength from a phase difference between the two        interfering arms of the first Mach-Zehnder interferometer;    -   two second photodetectors supported by the substrate and located        to receive the two second optical output interferometer signals,        respectively, wherein the two second photodetectors produce        third and fourth detector signals, respectively, and wherein        each of the third and fourth detector signals varies as a cosine        function of the optical frequency corresponding to the input        optical wavelength; and    -   a processing module coupled to receive the first, second, third        and fourth detector signals and operable to process the first,        second, third and fourth detector signals to determine a change        in the optical frequency of the input light.

Item 2. The device as in Item 1, wherein each of the first and secondMach-Zehnder interferometer includes at least one phase control devicecoupled to one of the two interfering arms to control the phasedifference between the two interfering arms.

Item 3. The device as in Item 1, wherein the at least one phase controldevice includes a heater to control the phase difference between the twointerfering arms.

Item 4. The device as in Item 1, wherein the substrate includes siliconor a silica.

Item 5. The device as in Item 1, comprising:

-   -   a polarization beam splitter supported by the substrate and        located in an optical path of the input light to split the input        light into a first input beam of the input light in a first        optical polarization and a second input beam of the input light        in a second optical polarization orthogonal to the first optical        polarization;    -   a first optical coupler supported by the substrate and located        to receive the first input beam and to split the first input        beam into the first and second portions of the input light that        are received, respectively, by the first and second Mach-Zehnder        interferometers in the first optical polarization;    -   a second optical coupler supported by the substrate and located        to receive the second input beam and to split the second input        beam into third and fourth portions of the input light in the        second optical polarization;    -   third optical waveguides integrated to and supported by the        substrate and coupled to form a third Mach-Zehnder        interferometer having two interfering optical arms and an input        optical port to receive the third portion of input light which        is split into the two interfering optical arms and an output        optical port to receive and combine light from the two        interfering optical arms to produce two third optical output        interferometer signals;|    -   two third photodetectors supported by the substrate and located        to receive the two third optical output interferometer signals,        respectively, wherein the two third photodetectors produce fifth        and sixth detector signals, respectively, and each of the fifth        and sixth detector signals varies as a sine function of the        optical frequency;    -   fourth optical waveguides integrated to and supported by the        substrate and coupled to form a fourth Mach-Zehnder        interferometer having two interfering optical arms and an input        optical port to receive the fourth portion of the input light        which is split into the two interfering optical arms, and an        output optical port to receive and combine light from the two        interfering optical arms to produce two fourth output        interferometer signals, wherein the fourth Mach-Zehnder        interferometer is structured to have a phase difference between        the two interfering arms different by one quarter of the input        optical wavelength from a phase difference between the two        interfering arms of the third Mach-Zehnder interferometer;    -   two fourth photodetectors supported by the substrate and located        to receive the two fourth optical output interferometer signals,        respectively, wherein the two fourth photodetectors produce        seventh and eighth detector signals, respectively, and wherein        each of the seventh and eighth detector signals varies as a        cosine function of the optical frequency; and    -   wherein the processing module is further coupled to receive the        fifth, sixth, seventh and eighth detector signals and operable        to process the fifth, sixth, seventh and eighth detector signals        to determine a change in the optical frequency of the input        light.

Item 6. The device as in Item 1, comprising:

-   -   a beam splitter supported by the substrate and located in an        optical path of the input light to split the input light into a        first input beam of the input light and a second input beam of        the input light;    -   an optical coupler supported by the substrate and located to        receive the first input beam and to split the first input beam        into the first and second portions of the input light that are        received, respectively, by the first and second Mach-Zehnder        interferometers;    -   third optical waveguides integrated to and supported by the        substrate and coupled to form a third Mach-Zehnder        interferometer having two interfering optical arms and an input        optical port to receive the second input beam which is split        into the two interfering optical arms and an output optical port        to receive and combine light from the two interfering optical        arms to produce two third optical output interferometer signals,        wherein the third Mach-Zehnder interferometer is structured to        have a phase difference between the two interfering arms that is        smaller than the phase difference in the two interfering arms of        the first and second Mach-Zehnder interferometer;| and    -   two third photodetectors supported by the substrate and located        to receive the two third optical output interferometer signals,        respectively, wherein the two third photodetectors produce fifth        and sixth detector signals, respectively.

Item 7. The device as in Item 1, comprising:

-   -   a beam splitter supported by the substrate and located in an        optical path of the input light to split the input light into a        first input beam of the input light and a second input beam of        the input light;    -   a first optical coupler supported by the substrate and located        to receive the first input beam and to split the first input        beam into the first and second portions of the input light that        are received, respectively, by the first and second Mach-Zehnder        interferometers;    -   a second optical coupler supported by the substrate and        structured to include two third optical waveguides that are        placed adjacent to each other to cause optical evanescent        coupling between the two third optical waveguides as a        directional optical coupler located to receive the second input        beam and to split the second input beam into third and fourth        optical signals;    -   two third photodetectors supported by the substrate and coupled        to receive the third and fourth signals output by the second        optical coupler, respectively, wherein the two third        photodetectors produce fifth and sixth detector signals,        respectively; and    -   wherein the processing module is further coupled to receive the        fifth and sixth detector signals and operable to process the        fifth and sixth detector signals to determine an absolute        frequency value of the optical frequency of the input light.

Item 8. A device for measuring an optical frequency of light,comprising:

-   -   a substrate;    -   a beam splitter supported by the substrate and located in an        optical path of the input light to split the input light into a        first input beam of the input light and a second input beam of        the input light;    -   a first optical coupler supported by the substrate and located        to receive the first input beam and to split the first input        beam into the first and second portions of the input light;    -   first optical waveguides integrated to and supported by the        substrate and coupled to form a first Mach-Zehnder        interferometer having two interfering optical arms and an input        optical port to receive the first portion of input light and        split the received first portion into different beams in the two        interfering optical arms and an output optical port to receive        and combine light from the two interfering optical arms to        produce three or more first optical output interferometer        signals in different phases relative to one another;|    -   three or more first photodetectors supported by the substrate        and located to receive the three or more first optical output        interferometer signals, respectively, wherein the first        photodetectors produce three or more first detector signals,        respectively, and each of the first detector signals varies as a        sine or cosine function of an optical frequency corresponding to        the input optical wavelength;    -   second optical waveguides integrated to and supported by the        substrate and coupled to form a waveguide device to receive the        second input beam from the beam splitter and to produce two        output signals of complementary wavelength responses;    -   two second photodetectors supported by the substrate and located        to receive the two output signals of the waveguide device,        respectively, wherein the two second photodetectors produce two        second detector signals, respectively, and wherein each of the        second detector signals varies as a cosine function of the        optical frequency corresponding to the input optical wavelength;        and    -   a processing module coupled to receive the first and second        detector signals and operable to process the first and second        detector signals to determine an absolute value of, and a change        in, the optical frequency of the input light.

Item 9. The device as in Item 8, wherein the waveguide device isstructured as a Mach-Zehnder interferometer having a larger freespectral range than a free spectral range of the first Mach-Zehnderinterferometer.

Item 10. The device as in Item 8, wherein the waveguide device isstructured as a directional coupler with a coupling ratio a slowfunction of optical wavelength having a larger free spectral range thana free spectral range of the first Mach-Zehnder interferometer.

Item 11. The device as in Item 8, wherein the output optical port is a2×3 multimode interference coupler with 3 output ports to produce threeinterference signals.

Item 12. The device as in Item 8, wherein the output optical port is a2×4 multimode interference coupler with 4 output ports to produce threeinterference signals.

Item 13. A device for measuring an optical frequency of light,comprising

-   -   a polarization beam splitter and rotator supported by the        substrate and located in an optical path of the input light to        split the input light into a first input beam of the input light        in a first optical polarization in a TE mode and a second input        beam of the input light in a second optical polarization        orthogonal to the first optical polarization in a TM mode, and        then rotate the second polarization of the TM mode into the        first polarization in the TE mode;    -   a first optical frequency sensing device coupled to receive the        first input beam from the polarization beam splitter to measure        an absolute value of, and a change in, the optical frequency of        the first input light; and    -   a second optical frequency sensing device coupled to receive the        second input beam from the polarization beam splitter to measure        an absolute value of, and a change in, the optical frequency of        the second input light,    -   wherein each of the first and second optical frequency sensing        devices is structured as a device for measuring an optical        frequency of light in Item 8.

Item 14. A device for light detection and ranging (LiDAR) on asubstrate, comprising:

-   -   a light source to produce an optical beam;    -   a beam splitter to split the optical beam into a first optical        beam and a second optical beam;    -   a beam scanning device located in an optical path of the first        optical beam to direct and scan the first optical beam to a        surrounding area for LiDAR sensing and to receive returned light        from objects in the surrounding area illuminated by the first        optical beam;    -   an optical detector module coupled to receive the returned beam        from the optical scanning device and to produce a detector        output signal for LiDAR sensing; and    -   an optical frequency detection device located to receive the        second optical beam to detect and measure a frequency variation        in the second optical beam caused by a frequency variation of        the optical beam produced by the light source,    -   wherein the optical frequency detection device is structured and        formed on a substrate according to one of Items 1 through 13;        and

wherein the measured frequency variation in the second optical beam isused to compute the fast Fourier transform of the detector output signalto obtain information of locations and speeds of the objects illuminatedby the first optical beam.

Item 15. The device as in Item 14, further comprising:

-   -   an electronic control module coupled to the light source and        operable to adjust an optical frequency of the optical beam        produced by the light source in response to the frequency        variation detected by the optical frequency detection device.

Item 16. A device for optical coherent tomography (OCT) on a substrate,comprising: a light source to produce an optical beam;

-   -   a beam splitter to split the optical beam into a first optical        beam and a second optical beam;    -   an OCT module coupled to receive the first optical beam from the        optical beam splitter to split the first optical beam into an        OCT reference beam and an OCT sampling beam for sampling a        target, and configured to cause optical interference between the        OCT reference beam and a returned OCT sampling beam from the        target for OCT detection; and    -   an optical frequency detection device located to receive the        second optical beam to detect and measure a frequency variation        in the second optical beam caused by a frequency variation by        the light source,    -   wherein the optical frequency detection device is structured and        formed on a substrate according to one of Items 1 through 13.

Item 17. The device as in Item 16, further comprising:

-   -   an electronic control module coupled to the light source and        operable to adjust an optical frequency of the optical beam        produced by the light source in response to the frequency        variation detected by the optical frequency detection device.

Item 18. A fiber sensing system, comprising:

-   -   a light source to produce an optical beam;    -   a sensing fiber coupled to receive the optical beam and        structured to include N different fiber Bragg gratings at        different locations in series in the sensing fiber for sensing,        wherein the N different fiber Bragg gratings are structured to        have different Bragg resonance wavelengths;    -   a wavelength division demultiplexing device coupled to receive        returned light from the N different fiber Bragg gratings in the        sensing fiber and operable to separate the received returned        light into N different optical beams at the different Bragg        resonance wavelengths, respectively;    -   a N×1 optical switch coupled with N input ports to receive light        from the wavelength division demultiplexing device with N output        ports and operable to switch or select one of the N different        optical beams at the different Bragg resonance wavelengths as an        optical switch output, one at a time;    -   an optical frequency detection device located to receive the        optical switch output from the optical switch and operable to        detect and measure a frequency variation in the received light,    -   wherein the optical frequency detection device is structured        according to one of Items 1 through 13.

Item 19. The device as in Item 18, further comprising:

-   -   an optical circulator that directs the optical beam from the        light source to the sending fiber and to direct light from the        sensing fiber to the wavelength division demultiplexing device.

Item 20. A fiber sensing system, comprising:

-   -   a light source to produce an optical beam;    -   a sensing fiber coupled to receive the optical beam and        structured to include N different fiber Bragg gratings at        different locations in series in the sensing fiber for sensing,        wherein the N different fiber Bragg gratings are structured to        have different Bragg resonance wavelengths;    -   a wavelength division demultiplexing device coupled to receive        returned light from the N different fiber Bragg gratings in the        sensing fiber and operable to separate the received returned        light into N different output optical beams at the different        Bragg resonance wavelengths, respectively;    -   N optical frequency detection devices each located to receive        one of the N output optical beams from the wavelength division        demultiplexing device and operable to detect and measure a        frequency variation in the received light from one of N Bragg        gratings,    -   wherein each of the optical frequency detection devices is        structured as an integrated device on a substrate according to        one of Items 1 through 13.

Item 21. The device as in Item 20, further comprising:

-   -   an optical circulator that directs the optical beam from the        light source to the sending fiber and to direct light from the        sensing fiber to the wavelength division demultiplexing device.

Item 22. A device for optical frequency domain reflectometry (OFDR),comprising:

-   -   a substrate;    -   a light source formed on the substrate to produce an optical        beam;    -   a first beam splitter formed on the substrate and coupled to        receive the optical beam to split the optical beam into a first        optical beam and a second optical beam;    -   a second beam splitter formed on the substrate and coupled to        receive at least a portion of the first optical beam to split        the received portion of the first optical beam into a probe beam        and a reference beam;    -   an optical port coupled to receive the probe beam from the        second beam splitter to direct the probe beam into an optical        fiber to cause light reflections or scattering inside the fiber        at different locations to generate returned probe light which        propagates back to the optical port and to the second beam        splitter, wherein the returned light carries information of        temperature, strain, stress at these locations in the optical        fiber;    -   an optical reflector located on or near the substrate to receive        the reference beam from the second beam splitter and to reflect        the reference beam back to the second beam splitter to interfere        with the returned probe light from the fiber at the second beam        splitter to produce an interference signal;    -   an optical detector module supported by the substrate and        coupled to receive the interference signal; and    -   an optical frequency detection device supported by the substrate        and located to receive the second optical beam to detect and        measure a frequency variation in the second optical beam caused        by a frequency variation in light of the optical beam produced        by the light source,    -   wherein the optical frequency detection device is structured        according to one of Items 1 through 13; and    -   wherein the measured frequency variation in the second optical        beam is used to compute a fast Fourier transform of the        interference signal to obtain information contained in the        returned probe light on temperature, strain, or stress as a        function of a location in the fiber.

Item 23. The device as in Item 22, further comprising:

-   -   an electronic control module coupled to the light source and        operable to adjust an optical frequency of the optical beam        produced by the light source in response to the frequency        variation detected by the optical frequency detection device.

Item 24. A photonic integrated interrogator chip for distributedinterferometric sensing, comprising:

-   -   a plurality of waveguides including an input waveguide to        receive input light, first, second, third, fourth, fifth, sixth,        and seventh waveguides structured to support light in transverse        electric (TE) polarization mode;    -   a first optical coupler to split the input light from the input        waveguide into a first optical beam in the first waveguide and a        second optical beam in the second waveguide;    -   an optical frequency detection device coupled to receive light        from the first waveguide to detect and measure an optical        frequency variation in the first optical beam caused by a        frequency variation by the input light to produce an optical        frequency variation signal, wherein the optical frequency        detection device is structured according to one of Items 1        through 13;    -   a second optical coupler coupled to receive light from the        second waveguide to split the second optical beam into a third        optical beam in the third waveguide and a fourth optical beam in        the fourth waveguide;    -   a waveguide delay line coupled to receive the third optical        beam;    -   a third optical coupler coupled to receive the third optical        beam from the waveguide delay line to split the third optical        beam into a fifth optical beam received by the fifth waveguide        and a sixth optical beam received by the sixth waveguide;    -   a first optical coherent receiver coupled to receive the fifth        optical bam from the fifth waveguide as a local oscillator        signal for coherent detection;    -   a second optical coherent receiver coupled to receive the sixth        optical beam from the sixth waveguide as a local oscillator        signal for coherent detection;    -   a polarization managing device coupled to receive the fourth        optical beam from the fourth waveguide which directs the fourth        optical beam to exit the interrogator chip from an output/input        port for optical sensing of one or more target objects and for        receiving a returned sensing beam containing a first        polarization and second polarization from the one or more target        objects, wherein the polarization managing device acts to        separate light in the returned sensing beam with the first        polarization into a first returned sensing beam in a seventh        waveguide supporting a first TE mode and light in the returned        sensing beam with the second polarization into a second returned        sensing beam in an eighth waveguide supporting a second TE mode;    -   the first optical coherent receiver coupled to the seventh        waveguide to receive the first returned sensing beam as an input        signal to the first optical coherent receiver to interfere with        the fifth optical beam to produce (1) a first detector signal        proportional to a sine function of a phase difference between        the fifth optical beam and the first returned sensing beam        and (2) a second detector signal proportional to a cosine        function of the phase difference between the fifth optical beam        and the first returned sensing beam;    -   the second optical coherent receiver coupled to the eighth        waveguide to receive the second returned sensing beam as an        input signal to the second optical coherent receiver to        interfere with the sixth optical beam to produce (1) a third        detector signal proportional to a sine function of a phase        difference between the sixth optical beam and the second        returned sensing beam and (2) a fourth detector signal        proportional to a cosine function of the phase difference        between the sixth optical beam and the second returned sensing        beam; and    -   wherein the first, second, third, and fourth detector signals        are processed in connection with the measured optical frequency        variation in the first optical beam to provide information on        amplitude, phase, motion, and location of the one or more target        objects.

Item 25. The photonic integrated interrogator chip as in Item 24,wherein the polarization managing device includes a fourth opticalcoupler connected to a ninth optical waveguide supporting both TE and TMmodes, an output/input port connected to the ninth optical waveguide,and a polarization beam splitter and rotator (PBSR).

Item 26. The photonic integrated interrogator chip as in Item 25,wherein the PBSR of the polarization managing device is configured tofirst separate light of the first polarization in a TE mode and light ofthe second polarization in a TM mode of the return sensing beam intolight beams in two optical paths and converts the TM mode into thesecond TE mode;

Item 27. The photonic integrated interrogator chip as in Item 24,comprising a fifth optical coupler connected to a first end of a tenthoptical waveguide supporting TE mode, and a 2-dimensional (2D) verticalgrating coupler (VGC) connected to the second end of the tenth opticalwaveguide, wherein the 2D VGC couples the fourth optical beam into anoptical fiber or into free space for sensing;

Item 28. The photonic integrated interrogator chip as in Item 27,wherein the 2D VGC is coupled to receive the return sensing beamcontaining both TE and TM polarization modes, directs the TE mode of thereturn sensing beam into the tenth optical waveguide, and directs the TMmode of the return sensing beam into an eleventh optical waveguideconnected to a TM to TE mode converter to convert the return sensingbeam in the TM mode into the second TE mode;

Item 29. The photonic integrated interrogator chip as in Item 24,wherein the first coherent receiver and the second coherent receiver are90° hybrid receivers.

Item 30. The photonic integrated interrogator chip as in Item 24,wherein the first coherent receiver and the second coherent receiverinclude 2×4 multimode interference couplers.

Item 31. The photonic integrated interrogator chip as in Item 24,comprising a first variable optical attenuator connected to the seventhwaveguide and a second variable optical attenuator connected to theeighth waveguide for adjusting the optical powers into the firstcoherent and the second coherent receivers.

Item 32. The photonic integrated interrogator chip as in Item 24,comprising a semiconductor optical amplifier connected to the fourthwaveguide to amplify light in the fourth optical beam.

Item 33. A photonic integrated interrogator chip for distributedinterferometric sensing, comprising:

-   -   a plurality of waveguides including an input waveguide to        receive input light, first, second, third, fourth, fifth, sixth,        and seventh waveguides structured to support light in transverse        electric (TE) polarization mode;    -   a first optical coupler coupled to the input waveguide to split        the input light from the input waveguide into a first optical        beam in the first waveguide and a second optical beam in the        second waveguide;    -   an optical frequency detection device coupled to receive light        from the first waveguide to detect and measure an optical        frequency variation in the first optical beam caused by a        frequency variation by the input light to produce an optical        frequency variation signal, wherein the optical frequency        detection device is structured according to one of Items 1        through 13;    -   a second optical coupler coupled to receive light from the        second waveguide to split the second optical beam into a third        optical beam in the third waveguide and a fourth optical beam in        the fourth waveguide;    -   a waveguide delay line coupled to receive the third optical        beam;    -   a third optical coupler coupled to receive the third optical        beam from the waveguide delay line to split the third optical        beam into a fifth optical beam received by the fifth waveguide        and a sixth optical beam received by the sixth waveguide;    -   a first 2×3 multimode interference coupler including a first        port coupled to receive the fifth optical bam from the fifth        waveguide and a second port coupled to the seventh waveguide;    -   a second 2×3 multimode interference coupler including a first        port coupled to receive the sixth optical beam from the sixth        waveguide and a second port coupled to the eighth waveguide;    -   a polarization managing device coupled to receive the fourth        optical beam from the fourth waveguide which directs the fourth        optical beam to exit the interrogator chip from an output/input        port for optical sensing of one or more target objects and for        receiving a returned sensing beam containing a first        polarization and second polarization from the one or more target        objects, wherein the polarization managing device acts to        separate light in the returned sensing beam with the first        polarization into a first returned sensing beam in a seventh        waveguide supporting a first TE mode and light in the returned        sensing beam with the second polarization into a second returned        sensing beam in an eighth waveguide supporting a second TE mode,        wherein the second port of the first 2×3 multimode interference        coupler is coupled to the seventh waveguide to receive the first        returned sensing beam as an input signal to the first 2×3        multimode interference coupler to interfere with the fifth        optical beam to produce a first set of 3 interference signals        which are 120° out of phase from one another;    -   the second port of the second 2×3 multimode interference coupler        is coupled to the eighth waveguide to receive the second        returned sensing beam as an input signal to the second 2×3        multimode interference to interfere with the sixth optical beam        to produce a second set of 3 interference signals which are 120°        out of phase from one another; and    -   wherein the first set and second set interference signals are        processed in connection with the measured optical frequency        variation in the first optical beam to provide information on        amplitude, phase, motion, and location of the one or more target        objects.

Item 34. The photonic integrated interrogator chip as in Item 33,comprising a fourth optical coupler connecting to a ninth opticalwaveguide supporting both TE and TM modes, an output/input portconnecting to the ninth optical waveguide, and a polarization beamsplitter and rotator (PB SR).

Item 35. The photonic integrated interrogator chip as in Item 34,wherein the PBSR first separates light of the first polarization in a TEmode and light of the second polarization in a TM mode of the returnsensing beam into two paths and converts the TM mode into the second TEmode;

Item 36. The photonic integrated interrogator chip as in Item 33,comprising a 5th optical coupler connecting to a first end of a 10thoptical waveguide supporting TE mode, and a 2-dimensional (2D) verticalgrating coupler (VGC) connecting to the second end of the 10th opticalwaveguide, wherein the 2D VGC couples the fourth optical beam into anoptical fiber or into free space for sensing;

Item 37. The photonic integrated interrogator chip as in Item 36,wherein the 2D VGC is coupled to receive the return sensing beamcontaining both TE and TM polarization modes, directs the TE mode of thereturn sensing beam into the 10th optical waveguide, and directs the TMmode of the return sensing beam into the 11th optical waveguideconnecting to a TM to TE mode converter to convert the return sensingbeam in the TM mode into the second TE mode;

Item 38. The photonic integrated interrogator chip of Item 24 or Item 31wherein the input waveguide receives light from different tunable lightsources with different characteristics for different applications.

Item 39. The photonic integrated interrogator chip of Item 24 or Item 31wherein the output/input port couples to an optical fiber for opticalfrequency domain reflectormetry (OFDR) applications.

Item 40. The photonic integrated interrogator chip of Item 24 or Item 31wherein the light beam exits from the output/input port and couples to ascanning optics to cause a 2D beam scan for optical coherence domaintomography (OCT) applications.

Item 41. The photonic integrated interrogator chip of Item 24 or Item 31wherein the light beam exits from the output/input port and couples to ascanning optics to cause a 2D beam scan for LiDAR applications.

While this patent document contains many specifics, these should not beconstrued as limitations on the scope of any disclosed technology or ofwhat may be claimed, but rather as descriptions of features that may bespecific to particular embodiments of particular disclosed technology.Certain features that are described in this patent document in thecontext of separate embodiments can also be implemented in combinationin a single embodiment. Conversely, various features that are describedin the context of a single embodiment can also be implemented inmultiple embodiments separately or in any suitable subcombination.Moreover, although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a sub combination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. Moreover, the separation of various system components in theembodiments described in this patent document should not be understoodas requiring such separation in all embodiments.

Only a few implementations and examples are described and otherimplementations, enhancements and variations can be made based on whatis described and illustrated in this patent document.

What is claimed is:
 1. A device operable based on measuring an opticalfrequency of light, comprising: a laser that operates to produce a laserbeam at a laser frequency and is operable to tune the laser frequency inresponse to a laser control signal applied to the laser; a laser controlcircuit that produces the laser control signal and is coupled to thelaser to apply the laser control signal to the laser; an optical beamsplitter located in an optical path of the laser beam to split the laserbeam into a first laser beam along a first path and a second laser beamalong a second path; an optical frequency sensing device coupled in thefirst path to receive the first laser beam from the optical beamsplitter to measure the laser frequency of the first laser beam based onmeasurements of sine and cosine functions of the laser frequency,wherein the optical frequency sensing device is further coupled to thelaser control circuit to provide information on the measured laserfrequency of the first laser beam, and wherein the laser and the lasercontrol circuit are coupled and interactive with each other to enablethe laser to stabilize or to change the laser frequency based on themeasured laser frequency of the first laser beam; and an optical portcoupled to in the second path to receive the second laser beam and tooutput the second laser beam as an optical output of the device.
 2. Thedevice as in claim 1, comprising: an optical interferometer located inthe second path between the optical port and the optical beam splitterto receive the second laser beam from the optical beam splitter andstructured to include two interfering optical arms and an opticalcoupler coupled to the two interfering optical arms to causeinterference between light in the two interfering optical arms tooptically interfere to produce optical interference, wherein one of thetwo interfering optical arms is coupled to the optical port to receivereturned light that is reflected from an object under illumination ofthe second laser beam so that the optical interference produces opticalinterference signals in the two interfering optical arms, respectively,each including information on the object carried within the returnedlight from the object; an optical detection module coupled to receiveand detect optical interference signals from the two interfering opticalarms to extract information on the object illuminated by the secondlaser beam.
 3. The device as in claim 2, wherein the optical detectionmodule is operable to allow extraction of information on the objectilluminated by the second laser beam based on measurements made viastabilizing the laser frequency of the second laser beam thatilluminates the object.
 4. The device as in claim 2, wherein the opticaldetection module is operable to allow extraction of information on theobject illuminated by the second laser beam based on measurements madevia scanning the laser frequency of the second laser beam thatilluminates the object.
 5. The device as in claim 1, wherein the opticalinterferometer located in the second path between the optical port andthe optical beam splitter includes: a first waveguide coupled to theoptical port to receive and to guide a first portion of light of thesecond laser beam from the optical beam splitter to the optical port toform one of the two interfering optical arms; a second waveguide coupledto receive and to guide a second portion of light of the second laserbeam from the optical beam splitter and to form the other of the twointerfering optical arms; an optical coupler coupled to the first andsecond waveguides to cause light in the first and second waveguides tointeract and to cause the optical interference; an optical reflectorcoupled to the second waveguide to reflect light as a reference opticalarm of the optical interferometer.
 6. The device as in claim 5, whereinthe optical detection module includes two photodetectors that detect theoptical interference signals from the two interfering optical arms,respectively.
 7. The device as in claim 1, wherein the optical frequencysensing device coupled in the first path to receive the first laser beamincludes: a substrate; first optical waveguides integrated to andsupported by the substrate and coupled to form a first Mach-Zehnderinterferometer having two interfering optical arms and an input opticalport to receive a first portion of the first laser beam that is splitinto the two interfering optical arms and an output optical port toreceive and combine light from the two interfering optical arms toproduce two first optical output interferometer signals;| two firstphotodetectors supported by the substrate and located to receive the twofirst optical output interferometer signals, respectively, wherein thetwo first photodetectors produce first and second detector signals,respectively, and each of the first and second detector signals variesas a sine function of the laser frequency of the first laser beam;second optical waveguides integrated to and supported by the substrateand coupled to form a second Mach-Zehnder interferometer having twointerfering optical arms and an input optical port to receive a secondportion of the first laser beam which is split into the two interferingoptical arms, and an output optical port to receive and combine lightfrom the two interfering optical arms to produce two second outputinterferometer signals, wherein the second Mach-Zehnder interferometeris structured to have a phase difference between the two interferingarms different by one quarter of the input optical wavelength from aphase difference between the two interfering arms of the firstMach-Zehnder interferometer; two second photodetectors supported by thesubstrate and located to receive the two second optical outputinterferometer signals, respectively, wherein the two secondphotodetectors produce third and fourth detector signals, respectively,and wherein each of the third and fourth detector signals varies as acosine function of the laser frequency of the first laser beam; and aprocessing module coupled to receive the first, second, third and fourthdetector signals and operable to process the first, second, third andfourth detector signals to determine a change in the laser frequency ofthe first laser beam.
 8. The device as in claim 7, wherein each of thefirst and second Mach-Zehnder interferometer in the optical frequencysensing device includes at least one phase control device coupled to oneof the two interfering arms to control the phase difference between thetwo interfering arms.
 9. The device as in claim 8, wherein the at leastone phase control device in the optical frequency sensing deviceincludes a heater to control the phase difference between the twointerfering arms.
 10. The device as in claim 7, wherein the substrate inthe optical frequency sensing device includes silicon or a silica. 11.The device as in claim 7, wherein the optical frequency sensing deviceincludes: a polarization beam splitter supported by the substrate andlocated in an optical path of the first laser beam to split the inputlight into a first input in a first optical polarization and a secondinput beam in a second optical polarization orthogonal to the firstoptical polarization; a first optical coupler supported by the substrateand located to receive the first input beam and to split the first inputbeam into the first and second portions of the input light that arereceived, respectively, by the first and second Mach-Zehnderinterferometers in the first optical polarization; a second opticalcoupler supported by the substrate and located to receive the secondinput beam and to split the second input beam into third and fourthportions of the input light in the second optical polarization; thirdoptical waveguides integrated to and supported by the substrate andcoupled to form a third Mach-Zehnder interferometer having twointerfering optical arms and an input optical port to receive the thirdportion of input light which is split into the two interfering opticalarms and an output optical port to receive and combine light from thetwo interfering optical arms to produce two third optical outputinterferometer signals;| two third photodetectors supported by thesubstrate and located to receive the two third optical outputinterferometer signals, respectively, wherein the two thirdphotodetectors produce fifth and sixth detector signals, respectively,and each of the fifth and sixth detector signals varies as a sinefunction of the optical frequency; fourth optical waveguides integratedto and supported by the substrate and coupled to form a fourthMach-Zehnder interferometer having two interfering optical arms and aninput optical port to receive the fourth portion of the input lightwhich is split into the two interfering optical arms, and an outputoptical port to receive and combine light from the two interferingoptical arms to produce two fourth output interferometer signals,wherein the fourth Mach-Zehnder interferometer is structured to have aphase difference between the two interfering arms different by onequarter of the input optical wavelength from a phase difference betweenthe two interfering arms of the third Mach-Zehnder interferometer; twofourth photodetectors supported by the substrate and located to receivethe two fourth optical output interferometer signals, respectively,wherein the two fourth photodetectors produce seventh and eighthdetector signals, respectively, and wherein each of the seventh andeighth detector signals varies as a cosine function of the opticalfrequency; and wherein the processing module is further coupled toreceive the fifth, sixth, seventh and eighth detector signals andoperable to process the fifth, sixth, seventh and eighth detectorsignals to determine a change in the laser frequency of the first laserbeam.
 12. The device as in claim 7, wherein the optical frequencysensing device includes: a beam splitter supported by the substrate andlocated in an optical path of the first laser beam to split the inputlight into a first input beam and a second input beam; an opticalcoupler supported by the substrate and located to receive the firstinput beam and to split the first input beam into the first and secondportions of the input light that are received, respectively, by thefirst and second Mach-Zehnder interferometers; third optical waveguidesintegrated to and supported by the substrate and coupled to form a thirdMach-Zehnder interferometer having two interfering optical arms and aninput optical port to receive the second input beam which is split intothe two interfering optical arms and an output optical port to receiveand combine light from the two interfering optical arms to produce twothird optical output interferometer signals, wherein the thirdMach-Zehnder interferometer is structured to have a phase differencebetween the two interfering arms that is smaller than the phasedifference in the two interfering arms of the first and secondMach-Zehnder interferometer; and two third photodetectors supported bythe substrate and located to receive the two third optical outputinterferometer signals, respectively, wherein the two thirdphotodetectors produce fifth and sixth detector signals, respectively.13. The device as in claim 7, wherein the optical frequency sensingdevice includes: a beam splitter supported by the substrate and locatedin an optical path of the first laser beam to split the first laser beaminto a first input beam light and a second input beam; a first opticalcoupler supported by the substrate and located to receive the firstinput beam and to split the first input beam into the first and secondportions of the input light that are received, respectively, by thefirst and second Mach-Zehnder interferometers; a second optical couplersupported by the substrate and structured to include two third opticalwaveguides that are placed adjacent to each other to cause opticalevanescent coupling between the two third optical waveguides as adirectional optical coupler located to receive the second input beam andto split the second input beam into third and fourth optical signals;two third photodetectors supported by the substrate and coupled toreceive the third and fourth signals output by the second opticalcoupler, respectively, wherein the two third photodetectors producefifth and sixth detector signals, respectively; and wherein theprocessing module is further coupled to receive the fifth and sixthdetector signals and operable to process the fifth and sixth detectorsignals to determine an absolute frequency value of the laser frequencyof the first laser beam.
 14. The device as in claim 1, wherein theoptical frequency sensing device coupled in the first path to receivethe first laser beam includes: a substrate; a beam splitter supported bythe substrate and located in an optical path of the first laser beam tosplit the input light into a first input beam and a second input beam; afirst optical coupler supported by the substrate and located to receivethe first input beam and to split the first input beam into first andsecond portions of the first input beam; first optical waveguidesintegrated to and supported by the substrate and coupled to form a firstMach-Zehnder interferometer having two interfering optical arms and aninput optical port to receive the first portion of the first input beamand split the received first portion into different beams in the twointerfering optical arms and an output optical port to receive andcombine light from the two interfering optical arms to produce three ormore first optical output interferometer signals in different phasesrelative to one another;| three or more first photodetectors supportedby the substrate and located to receive the three or more first opticaloutput interferometer signals, respectively, wherein the firstphotodetectors produce three or more first detector signals,respectively, and each of the first detector signals varies as a sine orcosine function the laser frequency of the first laser beam; secondoptical waveguides integrated to and supported by the substrate andcoupled to form a waveguide device to receive the second input beam fromthe beam splitter and to produce two output signals of complementarywavelength responses; two second photodetectors supported by thesubstrate and located to receive the two output signals of the waveguidedevice, respectively, wherein the two second photodetectors produce twosecond detector signals, respectively, and wherein each of the seconddetector signals varies as a cosine function of the laser frequency ofthe first laser beam; and a processing module coupled to receive thefirst and second detector signals and operable to process the first andsecond detector signals to determine an absolute value of, and a changein, the laser frequency of the first laser beam.
 15. The device as inclaim 14, wherein the waveguide device in the optical frequency sensingdevice is structured as a Mach-Zehnder interferometer having a largerfree spectral range than a free spectral range of the first Mach-Zehnderinterferometer.
 16. The device as in claim 14, wherein the waveguidedevice in the optical frequency sensing device is structured as adirectional coupler with a coupling ratio a slow function of opticalwavelength having a larger free spectral range than a free spectralrange of the first Mach-Zehnder interferometer.
 17. The device as inclaim 14, wherein the output optical port in the optical frequencysensing device is a 2×3 multimode interference coupler with 3 outputports to produce three interference signals.
 18. The device as in claim14, wherein the output optical port in the optical frequency sensingdevice is a 2×4 multimode interference coupler with 4 output ports toproduce four interference signals.
 19. A device for light detection andranging (LiDAR) on a substrate, comprising: a light source to produce anoptical beam; a beam splitter to split the optical beam into a firstoptical beam and a second optical beam; a beam scanning device locatedin an optical path of the first optical beam to direct and scan thefirst optical beam to a surrounding area for LiDAR sensing and toreceive returned light from objects in the surrounding area illuminatedby the first optical beam; an optical detector module coupled to receivethe returned beam from the optical scanning device and to produce adetector output signal for LiDAR sensing; and an optical frequencydetection device located to receive the second optical beam to detectand measure a frequency variation in the second optical beam caused by afrequency variation of the optical beam produced by the light source,wherein the optical frequency detection device is structured and formedon a substrate according to claim 1; and wherein the measured frequencyvariation in the second optical beam is used to compute a fast Fouriertransform of the detector output signal to obtain information oflocations and speeds of the objects illuminated by the first opticalbeam.
 20. A device for optical coherent tomography (OCT) on a substrate,comprising: a light source to produce an optical beam; a beam splitterto split the optical beam into a first optical beam and a second opticalbeam; an OCT module coupled to receive the first optical beam from theoptical beam splitter to split the first optical beam into an OCTreference beam and an OCT sampling beam for sampling a target, andconfigured to cause optical interference between the OCT reference beamand a returned OCT sampling beam from the target for OCT detection; andan optical frequency detection device located to receive the secondoptical beam to detect and measure a frequency variation in the secondoptical beam caused by a frequency variation by the light source,wherein the optical frequency detection device is structured and formedon a substrate according to claim
 1. 21. A device operable based onmeasuring an optical frequency of light, comprising: a broadband lightsource that operates to produce a probe beam at a range of opticalfrequencies; an optical circulator coupled in an optical path of theprobe beam from the broadband light source to direct the probe beam atan optical input/output port coupled to a fiber line and to receivereturned light from the fiber line and to direct the returned light to anew optical path different from the optical path of the probe beam whenpropagating to ward to the optical circulator; a wavelength divisiondemultiplexer located to receive the returned light from the opticalcirculator and to split the returned light into different returned beamsat different optical frequencies within the range of optical frequenciesproduced by the broadband light source; an optical switching devicelocated to receive the different returned beams at different opticalfrequencies from the wavelength division demultiplexer and structured tooperate to allow one beam to pass through at a time so that thedifferent returned beams at different optical frequencies from thewavelength division demultiplexer are directed to pass through theoptical witching device one at a time; an optical frequency sensingdevice coupled to receive light of the different returned beams atdifferent optical frequencies from the optical switching device thatpasses through the optical switching device and operable to measure anoptical frequency of the light that passes through the optical switchingdevice based on measurements of sine and cosine functions of the opticalfrequency of the light that passes through the optical switching deviceto enable measurements of the optical frequency of each of the differentreturned beams at different optical frequencies produced by thewavelength division demultiplexer; and a processor to receive themeasurements of the optical frequencies of the different returned beamsfrom the fiber line to extract information on the fiber line, includinga temperature or strain at different locations of the fiber line.
 22. Adevice operable based on measuring an optical frequency of light,comprising: a broadband light source that operates to produce a probebeam at a range of optical frequencies; an optical circulator coupled inan optical path of the probe beam from the broadband light source todirect the probe beam at an optical input/output port coupled to a fiberline and to receive returned light from the fiber line and to direct thereturned light to a new optical path different from the optical path ofthe probe beam when propagating to ward to the optical circulator; awavelength division demultiplexer located to receive the returned lightfrom the optical circulator and to split the returned light intodifferent returned beams at different optical frequencies within therange of optical frequencies produced by the broadband light source; aplurality of optical frequency sensing devices coupled to receive thedifferent returned beams at different optical frequencies, respectively,one device per returned beam, wherein each optical frequency sendingdevice is operable to measure an optical frequency of the light based onmeasurements of sine and cosine functions of the optical frequency ofthe light so that measurements of the optical frequencies of thedifferent returned beams can be measured; and a processor to receive themeasurements of the optical frequencies of the different returned beamsfrom the fiber line to extract information on the fiber line, includinga temperature or strain at different locations of the fiber line.
 23. Aphotonic integrated interrogator chip for interferometric sensing of anobject, comprising: an optical input port to receive input laser light;a waveguide coupled to the optical input port to guide the input laserlight received at the optical input port; an optical input/output portseparated from the optical input port and coupled to the waveguide toreceive a portion of the laser light in the waveguide and to output thereceived portion as probe light for optical sensing of an object and toreceive returned probe light from the object; a first optical couplercoupled to the waveguide to split a first portion of the laser light outof the waveguide into a first optical beam; an optical frequency sensingdevice coupled to receive the first optical beam from the first opticalcoupler and operable to measure an optical frequency of the firstoptical beam based on measurements of sine and cosine functions of theoptical frequency of the first optical beam; a second optical couplercoupled to the waveguide to split a second portion of the laser lightout of the waveguide into a second optical beam as an optical referencebeam for optical interference operations performed within the photonicintegrated interrogator chip; a first optical coherent receiver coupledto receive a first portion of the optical reference beam output by thesecond optical coupler and structured to include a first opticalinterferometer to cause the first portion of the optical reference beamto optically interfere with a first portion of the returned probe lightfrom the object to produce two first optical interference signals; asecond optical coherent receiver coupled to receive a second portion ofthe optical reference beam output by the second optical coupler andstructured to include a second optical interferometer to cause thesecond portion of the optical reference beam to optically interfere witha second portion of the returned probe light from the object to producetwo second optical interference signals; four optical detectors coupledto receive the two first optical interference signals and the two secondoptical interference signals, respectively, one signal per detector,wherein the measurement of the optical frequency of the first opticalbeam split from the laser light and information of the two first opticalinterference signals and the two second optical interference signals areused for extracting information of the object illuminated by the probelight output by the optical input/output port.
 24. The photonicintegrated interrogator chip as in claim 23, further comprising anoptical polarization device coupled between the optical input/outputport and the first and second optical coherent receivers to split thereturned probe light from the object into the first portion of thereturned probe light received by the first optical coherent receiver andthe second portion of the returned probe light received by the firstoptical coherent receiver while rendering the first and second portionsin a common optical polarization.